- Types of Zircon
- Formation of Zircon
- Sources of Zircon
- Fashioning of Zircon
- Physical & Optical Properties of Zircon
- Test and Identification of Zircon
- Valuation of zircon
Many gem species have certain varieties that appear on the market only after they have been treated in some way to make them more attractive or saleable than in their natural state. Zircon (pronounced ZUR-conn) is a species that would undoubtedly be little known to the modern jeweler, if heat treatment were not used to convert the rather dull, natural stones into colors that are virtually unknown in nature. To be sure, one occasionally sees yellow-brown, yellow-green and cloudy dark-green stones that are probably untreated. However, their use in jewelry is very restricted and would be considered by most jewelers to be of interest only to avid collectors, jewelers are usually acquainted only with the heat-treated colorless and blue stones.
Both blue and colorless stones occur naturally, but at present they are not found in sufficient quantity, and rarely in good enough quality, for commercial purposes. Actually, natural colorless zircons, as well as the heat-treated colorless stones, have been known for years. More than one hundred years ago, authors mentioned colorless zircons as possible substitutes for diamond. It was not until after the turn of the present century, however, that this gem mineral gradually became popular, when material discovered in the Mongka district, Indo-China (now Cambodia), was found to change to blue when heat treated. This was not true of material from any of the previously known deposits. Similar material was discovered later in Siam (now Thailand). About 1920, several dealers recognized the possibilities of zircon, and they financed a number of cutting plants in the Orient to cut stones to their specifications. The result was the popular modified brilliant cut that has been used for the majority of zircons since that time. Although Ceylon was the first important source of both naturally colored and colorless stones, it was not known for blue zircons. For at least four hundred years, the colorless stones were known as "Matara" or "Matura diamonds," from a district of that name in Ceylon.
Two descriptive terms have been associated with the name zircon, neither of which is very specific, and today their use is diminishing. HYACINTH, or JACINTH, has been used for reddish-brown natural zircon (as well as for hessonite garnet), and JARGOON has been used for pale yellow to colorless stones from Ceylon. The name zircon is of doubtful origin. It may have come to us through the French from the Arabic "zarkum" meaning "cinnabar" or "vermilion" or from the Persian "zargun" meaning "gold colored". Both could allude to the color of natural crystals, although the latter would seem a better description. The word zircon was first used by Weiner in 1783 for crystals from Ceylon. Jargoon (or, as it is sometimes spelled, jargon) probably has the same origin as zircon. Hyacinth was the name of a youth in Greek mythology who was slain; the wild hyacinth flower supposedly sprang from his spilled blood. Since wild hyacinth flowers in Greece are blue to violet blue. Pliny, in the first century A.D., compared the stone hyacinth with amethyst. Other ancient writers in alluding to hyacinth are surely describing blue to violet sapphires.
It is recommended that zircons should be described by their color, rather than by specific variety names. Thus, we have white for colorless, blue, orange, brown, red, green, and yellow zircons. It can be seen by this that the range of color includes all but the violet portion of the spectrum, although long exposure to X-rays or electron bombardment has been known to produce a temporary violet or purple color in some stones.
In addition to being far from clear cut in history and terminology, zircon is not simple mineralogically. It is recognized that there are three more or less distinct types of this mineral. These types have been given several, designations, such as alpha, beta and gamma; for clarity, however, we will refer to them as high, medium and low types, because their properties vary in this way.
In this regard, zircon is unique among gem minerals. Although the chemical composition, zirconium silicate, is fairly simple, certain alterations of the high type account for lower properties in the medium and low types. The three types were first described in 1903.
In addition to the elements zirconium, silicon and oxygen, which make up the chemical formula of zircon, small amounts of many other elements are usually present; e.g., zinc, calcium, iron, copper, titanium, radioactive elements such as uranium and thorium, and rare earths. The metallic elements hafnium (named for Hafnia, an old name for Copenhagen) was first isolated in 1922 from a Norwegian zircon.
In certain areas, beach sands contain a significant proportion of zircon grains. In some sands, the proportion is great enough to make mining feasible. In Florida and Australia, zircon is separated from sand by gravity concentration methods. The ore is used in the manufacture of refractories, high-temperature industrial ceramics, and chemicals, it is also used as foundry sand and as the source of the metal zirconium.
By far the most important type of zircon to the trade is the high-property type from Cambodia and Thailand, where it occurs as waterworn pebbles or stones with traces of tetragonal crystal forms in colors ranging from pale yellow brown to deep red-brown. When heated, colorless, blue and golden-yellow colors are produced. However, it has been ascertained that stones from different mines will produce different proportions of these colors.
The heat treatment is carried out by placing about one kilogram (5000 carats) of pebbles and crystals in a closed fireclay crucible and surrounding the crucible with coals in a primitive stove, much like the ones used in many Bangkok households. The stoves are fitted with suitable apertures and grates but no pyrometers (thermometers) are used. A funnel-like stovepipe or another stove on top of the first can be added to increase the draught, if needed the first heating in a reducing atmosphere turns many of the stones an acceptable blue color, although the proportion of worthless stones is said to be approximately 70%. Pale but clear stones are returned to the stove a second time in a perforated crucible, thus producing an oxidizing atmosphere, and many are changed to colorless, yellow, orange, etc. Further heat treatment may be employed, and some stones may thus have been heated five or six times, or even more.
Several authors have stated that complicated use of of chemicals was needed along with the heat treatment to produce white stones. Experiments in Western laboratories and more accurate information from visitors to the cutting areas discount the need for chemicals. In fact, one can demonstrate the effect of heat on a brown crystal by holding it in the flame of a blowpipe, gas jet, or even an alcohol lamp. Within five minutes a crystal of a carat or two will become colorless upon cooling. Similarly, a pale-blue stone might turn a darker blue with the same treatment, although since the heating is difficult to control, it may become greenish or lose all color. Thus, the experiment is not suggested as a remedy for changing faded stones to their original saleable.
Some investigators have indicated that a reducing atmosphere along will produce a blue color. The reason for the apparent discrepancy seems to be due to the fact that stones from different mines behave differently. Further, it has been found that some of the mines have produced much better colorless stones and stones that are less apt to fade or revert toward the original color. One mine was famed for its colorless stones, which were never troublesome in this manner. This mine is depleted. This perhaps explains part of the reason why some shipments of zircons give dealers in this country little trouble and why others tend to revert, even when kept in the darkness of a safe.
The rough stones are generally purchased by dealers who take them to Bangkok, where native lapidaries heat, cut and polish them with remarkable skill. Many of the more conscientious dealers subject the cut stones to strong sunlight for at least forty-eight hours in a well-ventilated place before shipment. The stones that are prone to change color are detected and rejected or reheated. Unfortunately, other dealers are not as careful. This was particularly true during the gem starved post World War II period, with the result that many zircons changed for the worse after delivery in the United States or Europe. The colors available in Bangkok include blue and colorless as the most important, with golden yellow and orange stones occasionally and garnet-red stones rarely. The unheated stones are seldom cut, even though their color may be a rather attractive dark red-brown to light brown.
Although many suggestions have been made regarding the causes for the color change and how to restore the desired color to the occasional stone that begins to revert toward its original appearance, it is fairly well proven that a zircon that once begins to alter will again do so regardless of how often the color is restored. Many importing firms in the United States grade their zircons carefully for color and store them for a period of time, regarding them occasionally and discarding those that have changed. After a year, those stones that are apparently color fast are sold, since the firms consider they have done their best to protect their customers.
In addition to changing the color, the heat treatment that most zircons undergo before entering the jewelry trade also alters the structure somewhat; as a result, they tend to chip or pit easily at facet junctions, even though they are actually somewhat harder than quartz. This accounts for the typical abraded facet junctions that are so common to zircons, even when they are handled and worn with care.
High-type (or, as it is sometimes called, normal
type) zircon has a high specific gravity, varying from approximately
to 4.8; a high refractive index of approximately 1.90-1.94 to 1.95-2.01, with a consequent birefringence near .06; dispersion factor that averages .038 (diamond 0.44), explaining why the colorless stone is an effective-diamond substitute; and a hardness of about 7 1/2. It is found in wellformed tetragonal crystals in many parts of the world, but only a few areas produce gem quality stones, and these are mostly water worn pebbles.
Low-type zircon occurs in gem quality only in Ceylon and only as rolled pebbles in the gem gravels without any trace of crystal faces. The color is usually green and usually tends to be a grayish to yellow-green with a somewhat cloudy texture; brown and orange stones are also known to occur. The stones are rarely sold as zircon in Ceylon, but usually are offered in parcels marked "tourmaline" (a word from the Singhalese meaning "mixed precious stones"), or sold as "green sapphires" to unsuspecting tourists.
Low zircon has been found to be composed of amorphous silica (SiO2) and amorphous or alternatively microcrystalline zirconia (ZrO2). Heating to 1450°C, usually does not alter it, beyond perhaps driving off a brownish cast in some stones. Overheating may change the stone to a worthless opaque condition. The properties seem to remain unchanged after heating. The refractive index varies from approximately 1.78 to 1.82, with just measurable birefringence. The hardness is about 6 and the specific gravity is 3.9 to 4.1. Dispersion is nil.
Gem material that is recognized as intermediate-type zircon comes only from Ceylon. It is found only as rolled pebbles in shades of greenish yellow to yellow-green, brownish green and greenish brown. As the name indicates, the specific gravity, refractive index and birefringence vary between those of the high and low types. Stones that approach the high type in specific gravity have the higher R.I. and birefringence and a more yellowish or brownish cast, instead of the purer green.
The most interesting characteristic of these intermediate stones is that heat treatment at about 1450°C causes the properties to increase toward those of the high type, some stones actually gaining the optical and physical properties of the high type. Since the bountiful supply of zircons from Cambodia (particularly the heat-treated blue stones) have come on the market, not much heat treatment is carried out in Ceylon, where only a muddy to cloudy-white color results from heating the intermediate-type stones. The occasional high-type brown stones may turn a fine orange color and resemble hessonite garnet. The untreated intermediate stones that do reach the market are usually sold under some other name or are sometimes imported for collectors. Occasionally, good yellow stones come on the market and are sometimes inadvertently sold as "golden sapphires".
It is now thought that the presence of radioactive elements such as uranium and thorium in zircon crystals at the time of formation account for the atomic derangement resulting in the breakdown of the normal structure into the nearly amorphous low type of zircon. This breakdown is fairly common in other minerals containing radioactive elements such as uranium ores, and is due to the incessant bombardment during millions of years by alpha particles emitted at great speed from the slowly dis-integrating radio active elements. Sorby, in the last century, along with Sir Arthur Church, was one of the first to study zircon under the spectroscope. In 1868, he examined some stones and detected such a spectacular absorption spectrum that he suspected a new element and proposed the name "jargonium" for it. (Ninety years previously, zirconium had been isolated and named for zircon by Klaproth.) Later, it was proven that uranium was responsible for the striking absorption spectrum seen in some zircons. Under the influence of heat, the process of disintegration is reversed in the intermediate stones, but it is assumed that too much radiation damage has been done in most low-type stones for heating to be successful in restoring the normal crystal structure. Why the colors change or just what causes the color in the first place is not agreed upon by scientists, although as we have seen, zircon frequently contains impurities, several of which might one day be discovered to be causative factors. It should be noted that there is no dangerous radiation from zircons today, and the subject of radioactivity should probably not be mentioned to a radiation conscious public.
An interesting occurrence in support of the radiation-damage
explanation for the intermediate and low types of zircon is the
occasional discovery of a crystal in which both types occur in
combination. A cut stone from Ceylon, described in 1952, was half green
(low type) and half orangey yellow (intermediate type). All properties
that could be measured showed that the
stone was indeed half low and half intermediate zircon. The assumption is that the crystal originally was formed with one half containing more radioactive elements than the other.
As a mineral, zircon is not rare. It is a common constituent of igneous rocks, such as syenite and granite, and of many metamorphic rocks, such as crystalline limestone and gneiss. In Norway, syenite is so rich in zircon that the rock has been given a type name of zircon-syenite . However, these rocks are not mined for either gem zircon or metallic zirconium. Because of the insolubility of zircon crystals and their toughness, compared to many other rock-forming minerals, they survive the breakdown of parent rocks and are therefore prominent constituents of gem bearing gravels in several areas of the world. These gravels are the only commercial source of gem zircons, since no source in matrix has ever been profitable to mine.
From ancient times the island of Ceylon has been the source of many colored stones. Zircon is one of them. Here it is found in the gem gravels with corundum, chrysoberyl, spinel, topaz, garnet (hessonite, almandite and rhodolite), tourmaline, quartz (rock crystal, amethyst and pale citrine), orthoclase moonstone, sphene, iolite, fibrolite, andalusite, diopside, rutile, apatite, sinhalite, komerupine and the exceedingly rare taaffeite.
The recovery methods used are very primitive. The mining is entirely in the hands of the Singhalese. Usually, an owner hires workers for their food and housing, plus a share of any yield beyond their cost to him. If the pay gravels are at or near the surface, mining is simple. An area of about four feet square, making room for two or three miners to work, is worked to the bottom of the pay zone, but this is not undertaken until the appropriate ceremonies beseeching the gods for success are conducted. This kind of mining is successful only if the gem bearing layer, locally called "illam," is near the surface. If the gravel is much deeper, the pits are dug about six feet square and from ten to twenty feet deep or even more. Unless the overburden is very strongly consolidated, the pit walls must be shored to prevent caveins. If a number of pits are dug close together, tunnels may be constructed connecting them. One disadvantage of a deep pit is that it is likely to be below the water table in most of Ceylon. This water is bailed out by an especially constructed winch with a number of buckets attached to a connecting rope with stone counterweight. The miners dig until they encounter either a blackish rock, known as "ralu ratta," or a rocky gravel of whitish color called "thiruvana". This is an indication to the miner that the next layer must contain gems he is near the illam. If they encounter a layer of clay after removing the illam with their baskets and the winch, they do not dig any farther, because in do so has proven fruitless.
When no more illam is obtainable, the owner chooses an
auspicious day for the washing and sorting. Until this time, the heap of
gravel is covered with leaves and well guarded. If the mine is near a
river, the gem gravels are transferred
to its bank and washed in special conical baskets. The lighter materials is swirled off and the heavier contents are then examined. In other areas, trenches are dug and filled with water for the washing process. Zircon is the densest gemstone recovered. The mine owner is always on the spot, since the miners do the washing and sometimes cannot resist the temptation to hide a good stone. Occasionally, the illam is found in a stream bed; in this case, the stream is dammed, except for about five feet in the middle, where a log is placed to control the water level. Using a long rake like implement, the miners pull the gravels toward the log until the Mem layer is reached. Then, either by wading in with baskets or by diving, the illam is collected and washed on the spot.
Mining is conducted mostly during the dry season, which is from March to November. At other times, the heavy rains fill the pits and trenches, making mining impossible. The mining is not done for zircon alone, for zircon, as mentioned previously, is only one of the many gem minerals found, several of which are more valuable to the miners and owners.
Certain areas and mines in Ceylon are known for the rather greater concentration of one gem mineral than another. Chrysoberyl cat's eye, for example, is more prevalent in one area than in another, and the same is true with certain sapphires and moonstones. No one area, however, is particularly noted for zircon. The most profitable mining centers are near the town of Ratnapura, near the southeast end of the island.
Only the actual mining is in the hands of the native Singhalese; the gem trade in Ceylon is carried on exclusively by Moors, ancestors of whom reached Ceylon centuries ago. Moorish lapidaries in the area of the mines cut many of the stones, which are then sold to dealers in Colombo. As was previously mentioned, the zircons among them are seldom sold as such, but are relegated to nixed parcels that are sold for little money, in as much as neither the cutting nor the colors of most are particularly desirable.
As stated before, the most important source of zircon to the trade is Cambodia, particularly in the Kha district, east of the Mekong River, in an area near the border between Cambodia and South Vietnam (indicated by the solid-black circle on the accompanying map). Mining methods are essentially the same as those in Ceylon, since in both areas the miners are searching for layers of subsurface alluvial gravel in which heavy minerals are concentrated. Pits must be dug to reach the gem gravels, and these are seldom more than eight to ten feet deep. One difficulty in some areas is the relatively drier climate, although along the banks of the rivers and streams, water is available for the washing. It is claimed that most of the mining is done by Burmese hill people, who discovered the deposits late in the last century. One difference between the mines of Ceylon and those of the Cambodia-Thailand region is that in the former many different species and varieties are found, whereas, except for one mine in Cambodia, only zircon is commercially important. The Kha mines are very remote and are seldom visited by Westerners. Those who brave the trip say they can buy rough more advantageously in Bangkok than at the mines. The Ceylon mines are readily accessible by train from Colombo.
Greenish-yellow to yellow-green zircons are found
occasionally in the Burmese ruby mines. No extensive trade in zircons
is carried on here, and those discovered are usually sold without treatment or, at times, without identification. These stones are unusual for their striking absorption spectrum in the spectroscope, where as many as forty or more lines may be seen.
No other commercial source of zircon exists, although cutable stones have been found in France as brilliant red crystals at Espaly-Saint-Marcel; in New South Wales, AusNo other commercial source of zircon exists, although cutable stones have been found in France as brilliant red crystals at Espaly-Saint-Marcel; in New South Wales, Australia; with diamond in kimberlite in South Africa; in Madagascar; in Russia; Mention should also be made of the gigantic red crystals that have been found in Canada in Renfrew county, Ontario. Some of these have weighed up to fifteen pounds and measured more than twelve inches long, although clear areas would cut stones little larger than one carat in weight.
Compared with most colored stones cut in the Orient, zircon is the only species for which standards were established to control cutting quality at the time the material was first promoted. As a result, dealers today would not consider buying poorly cut stones. This factor has been largely responsible for the success of zircon in jewelry. The cuts employed are very much like those for diamond. The most successful is the round brilliant, because, as with diamond, it allows the high refractive index and dispersion to be manifested as brilliancy and fire. The attempts to popularize marquise and other fancy cuts have not been successful, since the stones seem dull compared with diamond, although small baguettes cut fuller than diamond baguettes are quite effective. Formerly, a type of cut recognized as the "zircon cut" was used extensively. This style utilized an extra eight facets around the culet. Today, examination of newly imported lots usually fails to disclose examples of this method of fashioning; instead, the standard 57-facet brilliant is used (no culet), especially for the blue and colorless stones. In general, in order to display the dispersion to best advantage, the crown of brilliant cut zircons is heavier than most brilliant cut diamonds, particularly since "spreading" has been so much in evidence with diamond in recent years.
In Ceylon, the usual "Ceylon" style of cutting is generally employed, which makes the best use of the material for weight but is woefully lacking in symmetry and brilliancy. One may therefore see cushion antique cuts, off-shaped tear drops, and unclassified off shapes. Sometimes the stones are deeper than they are wide. Thus, matching stones is impossible without re-cutting. Standard mountings that are used for other inexpensive stones can rarely be used. For this reason, zircon in Ceylon suffers in popularity from the hidebound cutting traditions even more than the more valuable species from Ceylon, which are cut primarily to save weight too. Re-cut in this country or Europe, some of Ceylon's green and golden yellow stones are very attractive.
Good heat-treated rough material for cutting in this country or Europe is very difficult for lapidaries to obtain. One rarely sees crystals that are worth cutting, even if it would pay financially to cut them here or in Europe. When parcels of rough white or blue material are seen, close inspection usually discloses that the stones are evidently rejects from Thailand, being either too cloudy or having inclusions that could not be removed economically. Parcels of low-quality mixed stones may be purchased occasionally from Ceylon. Usually, a high percentage of the stones will be medium-and low-type zircon, together with brown chrysoberyl, dark sapphire and spinel, and sometimes the rare sinhalite and komerupine.
In Bangkok, the cutting of zircons is not concentrated in a few large shops, as the more or less standardized cutting would suggest; Instead, the cutting takes place in many shops having a few polishing wheels each. Therefore, the methods of working the rough are not necessarily standardized, just as the heating is not carried out under identical conditions. The rough stones are seldom large enough to require sawing.
They rare roughed out on vertical carborundum hand wheels, then smoothed on fine-grit horizontal carborundum wheels. Facets are cut and polished on steel or iron laps with bort as the abrasive.
Lapidary work on zircon in this country is confined largely to re-polishing worn stones (if they are large enough to warrant the cost) or re-cutting off-shaped Ceylon stones. Care must be exercised during the sawing operation, because zircon tends to clog the diamond saw. Facet grinding may be done on a copper lap with diamond powder or on a lead wheel with carborundum powder. The proportions usually recommended are one-third above the girdle and two-thirds below. An effective cut is produced when the main crown facets have an angle with the girdle of about 40° and the inclination of the pavilion facets is at 41°. Ideal diamond angles are 341/2° for the main crown facets and 40 3/4° for the pavilion facets. Examination of a number of commercially cut zircons from Bangkok indicates at least as good adherence to good proportions as one finds in most diamonds. Polishing is usually accomplished most satisfactorily with Linde A powder. Some zircons, especially the low type, may present zones of uneven hardness. With such stones, polishing may proceed normally with damp polishing Powder; however, when the soft zones are encountered, the powder should be permitted to become almost dry and the lap speed should be reduced considerably.
|Chemical Composition||Zirconium Osilicate (pure zircon), expressed by the formula ZrSiO4|
|Crystallographic Character||Tetragonal system, Habit tetragonal prism and bipyramid. Gems are usually found as waterworn pebbles.|
|Hardness||High and medium types : 7 to 7 1/2. Low type : 6.|
|Toughness||Heat-treated stones: Poor, Non treated stones; poor to fair.|
|Specific Gravity||High type: 4.6 to 4.8; usually, near 4.7. Medium type : 4.1 to 4.6. Low type : 3.9 to 4.1|
|Characteristic Inclusions||Low-type green stones usually show very strong repeated twinning or zoning that may impart a milky appearance at some angles of illumination. Skeltetal, angular inclusions (see photo) are common in the same type. Minute, white cottony inclusions are seen in some blue and many colorless stones.|
|Degree of Transparency||Transparent to semitransparent.|
|Luster||Fracture surfaces are subadamantine; polished surfaces are adamantine.|
|Refractive Index||High type: 1.90-1.94 to 1.95-2.01.
Low type: 1.78-1.783 to 1.81-1.815.
Medium zircon is intermediate between the high and low types
|Birefringence||High type : .04 to .06.
Medium type: .01 to 0.4.
Low type : very slight.
|Optic Character||Uniaxial positive.|
|Pleochroism||Blue: strong blue and grayish yellow to colorless.
Green: very weak green and yellow-green.
Orange and brown: weak to distinct purplish brown and brownish yellow.
Red : distinct reddish purple and purplish brown
|Dispersion||High and medium types: averages .038
Low type : very low
|X-Ray, Fluorescence||Variable; most glow white to bluish.|
|Color-Filler Reaction||Green stones only may show slightly red.|
|Absorption Spectra||May show as many as forty or more absorption lines and hands for some green Burma stones to none for some reds and browns. Intermediate-type Ceylon stones usually show approximately twelve bands. Some heated blue and white stones will show a 6535 A.U. Line only. Low-type green zircons usually exhibit only a broad, vague band in the 6535 A.U. area.|
Effects Caused by:
|Heat||Infusible before the blowpipe or the jeweler's torch. High and some medium types may change to colorless, blue, golden yellow or red-brown.|
|Acids||Not attacked, except when powdered.|
|Irradiation||Under X-rays and in the absence of heat, previously heat treated stones may turn an attractive violet to purple-brown color. Others may darken to a brown (original?) color. Electron bombardment in a Van de Graaff generator turns colorless and blue heat-treated stones a deep red- brown. Those in the center of the beam retain their color, presumably the original color; others fade through orange-red, orange, yellowish brown to colorless, if not kept in the dark. Stones that received attendant X-radiation turn red- brown but fade through purple-brown to purple, violet and finally to colorless, if not kept in the dark.|
Since zircon occurs in a wide range of colors, it may be confused with almost any transparent to semitransparent gemstone with a similar color. To the casual observer, a well-cut and scrupulously clean zircon may be confused with a diamond of the same color. Since zircons that are able to masquerade as diamonds will be either the high or medium type, the most obvious characteristic is the apparent doubling of facet junctions, inclusions, scratches or dirt observed through the stone. Since the amount of doubling depends on the orientation of a cut stone, it may be necessary to make observations in several directions. Because diamond is singly refractive, the observance of any doubling eliminates diamond as a possibility.
Of the other transparent stones that might be confused with high or medium zircon on the basis of appearance alone, all but the rare gemstone sphene and synthetic rutile may be eliminated on the basis of double refraction, S.G. (for unmounted stones) and R.I., which is higher than the refractometer scale (1.81). The S.G. of sphene is approximately 3.52, which is lower than for any zircon. The easy parting and cleavage of sphene is observed frequently as incipient cracks in a cut stone. The absorption spectrum of a zircon that resembles sphene would almost certainly separate the stones. Synthetic rutile displays such a high degree of dispersion that few persons are likely to confuse it with zircon. Also, the strong doubling in some directions in synthetic rutile is far greater than zircon and would be unmistakable to anyone who has observed it in even a few zircons. Stones represented as synthetic zircon are almost sure to be synthetic spinel or, rarely, synthetic sapphire. No difficulty in identifying them should occur, if recourse is had to the refractometer and polariscope.
The low-type green zircon can be confused by sight alone with various green stones, such as tourmaline, diopside, demantoid garnet, peridot and sapphire. Again, the refractometer and polariscope will suffice to make identification positive in almost every case. Green sapphire may pose a problem. Although corundum and synthetic corundum are amazingly consistent in R.I., averaging as they do 1.762-1.770, there are two exceptions: the very dark-red synthetic that is used to imitate almandite garnet, and natural green sapphire. Both may read as high as 1.772-1.780 in R.I. Rarely, low zircon may descend to 1.78; however, at that level, there is no visible birefringence in zircon. Since it may have the same S.G. and similar appearing inclusions, it can be seen that instrument tests must be made with care. Of course, the hardness of the zircon is considerably less, but a less drastic test than hardness is to observe the absorption spectrum, which, as we have seen, is distinctive for this type.
Occasionally, backed rose-cut colorless zircons appear in Indian
(Hindu) jewelry. They may be interspersed with rose-cut diamonds and the
characteristic enamel and colored stones that are so typical of this
kind of jewelry. Since R.I. and S.G. cannot be taken, the clue is to
observe the stones under high magnification for double refraction. If
doubling is seen in these small, flat stones, it is fairly certain that
they are zircon, since no other colorless doubly refractive stone in
such small sizes will show pronounced doubling. Because blue zircon may
have the hue of good aquamarine (but the latter are hard to find
in good color in small sizes), one occasionally encounters aquamarines set with matching small zircons. Again, the observation of strong doubling in small stones is virtually conclusive, for no other aquamarine blue stone shows such strong doubling in such small sizes.
In the mixed parcels of Ceylon gems, two stones that are frequently confused on sight are brownish-yellow zircons and sinhalite, both of which show a noticeable doubling of back facet junctions. One dealer, upon learning of the "nevi" stone, sinhalite, bought parcels of Ceylon zircons abroad for about $20 per carat. There were nine sinhalites about of thirty-four stones he thus obtained from unsuspecting sellers. He needed the sinhalites, since he has a large clientele of collectors.
Of the two color varieties of zircon that are the most popular, colorless and fine blue, the latter is the most valuable. This color is found in no other gemstone, except, ironically, some of the more recent colors of diamond produced experimentally by atomic bombardment. Blue zircons should be examined by daylight to determine if the blue is pure and not modified by a yellowish cast, particularly in one dichroic direction. If a yellowish cast is present, it indicates a stone that may be reverting to its original color. Although the blue of fine zircons is usually slightly greenish, some approach the color of fine aquamarine; these bring the highest prices, if the color is evenly distributed.
Colorless zircons should be as transparent as a fine diamond and free from any grayish or brownish cast. Although the stones with a brownish cast may be reheated to restore colorlessness and fine transparently, the change is usually short lived. Some actually seem to display greater dispersion and a higher luster for a few hours after heating, but return to the more normal condition afterwards.
Other colors of zircons should be as transparent as the color will allow. Purity of color, evenness, and clarity of texture should be observed. Dull stones of low intensity or poor brilliancy have little appeal and are of little value. Even the better fancy colors (i.e., those other than blue and colorless) have had little demand, due to the lack of appreciation on the part of both buyer and seller. The fine flame-colored stones seem to have enough appeal to sell readily, however.
Flawless or nearly flawless zircons have been available in sufficient quantities so that there has been little demand for flawed material, specially in colorless and blue stones. These should have no blemishes, at least when judged with the unaided eye. Stones should be observed for the characteristic abrasions along facet junctions. Zircons that have been jumbled together in papers without being wrapped individually will show the abrasions. If visible to the unaided eye, the stones should be rejected or Tepolished, if the value warrants. It is well to reject stones that have a cloudy texture, regardless of their color.
The beauty of zircons is primarily the result of high R.I. and dispersion, together with the adamantine luster of well- polished stones. The cutters in Bangkok seem to understand this, since they keep the cutting fairly well standardized. To attempt to "swindle" the stones to get more from the rough at the expense of beauty, as is so commonly done with diamond, would only defeat their purpose in cutting. However, blue and colorless zircons with flat crowns or other departures from the ideal are occasionally encountered, and they should be judged less valuable than well-made stones. In a multi- stone piece of jewelry, the stones should be cut to about the same proportions, so that the design is pleasing and there are no "dead" stones. With cuts other than the brilliant, attention should be paid to the over-all brilliancy. If , as is seen in marquise cuts, particularly, there is a large dead area in the center, the stone will not give satisfaction when worn. This is true particularly in ring stones, where dirt frequently collects on the back of stones and shows through. This, in fact, is one disadvantage of even the best zircon when compared with diamond, and is much worse for a stone with a dead area. Ceylon-cut stones are usually judged on the basis of their value after re-cutting to proper proportions, although an attractive stone may occasionally be encountered.
Because of the high birefringence of high-type zircon, stones with the optic axis parallel to the table, or nearly so, will show a fuzziness caused by the doubling of the back facets. This is particularly true in larger sizes. If this is to be avoided, the stones should be cut with the optic axis perpendicular to the table. To judge this factor, the stones should be observed directly through the table. Those with no doubling in this direction are more attractive, although they may not necessarily command a better price, since color and other factors must also be considered.
Orientation also affects the color of blue stones, because some will show a desirable blue across one area of the stone alternating with distinctly lighter blue or, worse, with a yellowish blue. The color should be as even as possible, although some slight variation is almost unavoidable. As with the corundum gems, fortunately, the best blue color is seen parallel to the optic axis. Therefore, when the stones are cut so that the optic axis is at right angles to the table, the best possible color will be visible, and the fuzziness noted above will be at a minimum.
Blue and colorless zircons are usually available in sizes from a few points in weight to nine or ten carats. At one time prior to World War II, 15 to 25 carat blue stones were available from many suppliers. Since the War, stones in excess of ten carats have been very scarce, especially in truly fine colors.