Tourmaline




multi color tourmaline

Although tourmaline (pronounced TOUR-mah-leen) was undoubtedly known to the ancients, since it occurred in many of the oriental mines that yielded other precious stones for the gem-loving Romans, there are no definite descriptions of it by the gem writers of the early periods. During the seventeenth century, Brazil exported long prisms of dark green tourmaline to Europe but called them "Brazilian emeralds" stating incorrectly that they were harder than true emeralds.

It was not until the early part of the eighteenth century, however, that an incident led to the discovery that these beautiful crystals had a strange property not possessed by emerald. According to the story, one warm summer day some children of Amsterdam, Holland, were playing with stones that had been brought home by Dutch navigators and noticed the odd effect produced on them by the sun's rays. The children's parents, summoned to view the phenomenon, were likewise astonished to find that the stones could attract or repel, with decided force, such light-weight substances as ashed and straws. Bemuse of this, the Dutchmen called them "aschentreckers," or "ash drawers."

For a number of years after the discovery of the green crystals in Brazil, there was some doubt about the true nature of the mineral. The black variety of tourmaline, a schorl, had been known in Europe for almost two centuries, but it was not until 1768 that Linnaeus, the noted Swedish naturalist, intimated the relationship of schorl to the green variety. Shortly thereafter, Delisle, the French scientist, proved conclusively that the green crystals had the same properties and form of crystallization as the black. It was, however, a long time before the disputants adopted the present name of tourmaline, which is derived from the ancient Singhalese word "turmali." The meaning of this term, "mixed precious stones" is applied to gems the identity of which the Ceylong gem merchants apparently are in doubt about fortunately, this-confusing usage has not spread to other parts of the world, although it seems still to be used by them for both rough and cut parcels.

Varieties and trade names of Tourmaline

Chemically, tourmaline is one of the most complex of all the gem minerals. Although it is generally considered a species, there are three major mineralogical types: the alkali, the iron, and the magnesium tourmalines, all of which are complex silicates of boron and aluminum. The differences that distinguish one type from another depend on the additional metallic ions present. The alkali tourmalines, which are of greatest interest geologically, may contain sodium, lithium and/or potassium; these are usually found in pegmatite dikes. Iron tourmalines are black, and the magnesium type is usually yellow or brown, but it may also be black.

Mineralogists have applied many variety names to tourmaline, including dravite, achroite, schorl, indicolite and rubellite. In addition to these terms, a number of others were used by mineralogists and colored-stone dealers that were even more confusing. For example, blue material was called "Brazilian sapphire" and the green was known as "Brazilian emerald" or "Ceylonese peridot". Since they serve no useful purpose and complicate the picture unnecessarily, it has been recommended that all specific variety names be abandoned. Almost ail of these useless and confusing names are gradually being dropped. Because tourmaline as a gem is so little known to the lay public, and to many jewelers, such obscure variety names tend to mislead and rob the species of needed recognition.

tourmaline

Red Tourmaline

The most valuable variety is red to purplish red to violetish red. Pale pink to dark red stones are available and in some demand; these are the colors that were once called rubellite. There are an almost infinite number of variations of both hue and tone in this classification. Some stones are brownish red, some orangey red, others violetish red, and still others more nearly violet or purple than red. Tones vary from light to dark.

The red variety is best termed red tourmaline. Although rubellite was the best known of the variety names, it adds nothing and has the drawback of creating the impression that it is some kind of ruby imitation. In this age of substitutes, such a term is sure to be little a natural stone. Red tourmaline is a much more satisfactory name. Usually, it more closely resembles rhodolite garnet than ruby. It almost never approaches the intensity of color that one expects in a fine quality ruby; moreover, the hue usually has a greater purplish content, often with a faint brownish cast. However, some exceptional stones are of an intensity more reminiscent of ruby than most other red gemstones.

Green Tourmaline

Include within this variety are stones varying from light green to dark bluish, yellowish or brownish green. They may be very pale or so dark that the color is visible by transmitted light only. The strong dichroism of most green tourmaline is such that the optic-axis direction is almost opaque; by heat treatment, however, the color of some can be improved and the absorption reduced. Very rarely, intensely colored stones appear on the market that resemble fine yellowish green emerald; unfortunately, however, stones of this quality are almost impossible to obtain. Since the usual commercial material is regarded as attractive to most persons, it can be used effectively for those who consider emerald out of their price range. Dark-green tourmaline is often called African or Transvaal tourmaline, even though it is almost certain to have been mined in Brazil.

Blue Tourmaline

Pure blue tourmaline is a rare gemstone; it is most frequently a dark violetish or greenish blue. If the intensity of color is sufficient to warrant, it may be fairly expensive, but this variety is more of a curiosity than a commonly encountered gemstone. It is sometimes called indicolite or indigolite. It has also been called "Brazilian sapphire" incorrectly. Occasionally, light-blue to light greenish-blue tourmaline is seen; this type of material is usually readily saleable, particularly those stones that resemble very fine aquamarine.

Yellow and Orange Tourmaline

Pure yellow or orange tourmaline is rarely encountered. More prevalent are light to very dark tones of yellowish brown to brownish yellow, which are reminiscent of sherry topaz and golden beryl; greenish brown to brownish green, which resemble andalusite; orangey brown to brownish orange; and greenish yellow. There is almost tourmaline is sometimes called dravite by mineralogists.

Colorless, or White Tourmaline

Colorless tourmaline, sometimes called achroite (from the Greek, meaning "without color" ), has little interest as a gemstone, except in cat's-eye material. It is rare in nature.

Black / schorl Tourmaline

Very rarely, the black variety of tourmaline is used in place of jet or black chalcedony in inexpensive stone set jewelry. This variety is known to mineralogists as schorl.

Multi colored Stones

Tourmaline crystals often show more than one color. Some will be one color at the base, another at the center, and a third near the apex (Figure I). In others, the interior portion will be one color and the peripheral zone another. When the central portion of the the crystal is pink and the periphery green, the combination is referred to as "watermelon" tourmaline. Particolored crystals are sometimes cut for gem purposes, but they are more curiosities than gemstones. Sometimes, one pink and one green stone will be cut from the same crystal.

Chatoyant Tourmaline (Tourmaline Cat's-Eye)

Some green tourmaline, and rarely other colors, may have sufficient cavities or needlelike inclusions to exhibit chatoyancy when cut in cabochon. These stones have little value, unless the eye is well defined. A few brownish green stones have been seen. Also seen is fine quality blue, blue green and red tourmaline cat's-eye in which the coarse needles were concentrated at the bottom of the cabochon, thus imparting to the stones a greater clarity and beauty not possessed by stones with the coarse hollow tubes throughout.

Alexandrite like Tourmaline

An extremely rare variety has been reported that exhibits a color change when observed in daylight and artificial light. The usual change is a yellowish to brownish-green daylight color and an orange-red or orangey-red color under artificial light.

Formation of Tourmaline

Tourmaline is formed under a variety of conditions, but from the gemological viewpoint, the type formed in pegmatite dikes is the most important. However, it is found in a number of other situations, all of which suggest the presence of hot gases and solutions acting under pressure. Like topaz, tourmaline is thought of as a pneumatolytic mineral for this reason; that is, one that results from the actions of hot gases and/or solutions, either in the late stages of cooling and crystallization of a magma or by the action on earlier-formed rocks of gases or solutions emanating from magmas.

Sources of Tourmaline

In recent years, the most important source of tourmaline has been, and is, the rick pegmatite dikes of Brazil. From this area (mostly in the State of Minas Gerais) comes fine material in a wide variety of colors. It has a collection of more than twenty different colors of Brazilian tourmaline, representing all of the major hues. Pegmatite dikes in Brazil are mined for a variety of strategic minerals, as well as for gemstones. The major tourmaline producing dikes are usually mined principally for that gemstone. Although red and green are the most popular colors, all cuttable material is marketable. Some of the dikes being mined in this country are decomposed sufficiently so that the large feldspar content has become more of a clay than a rock. For the most part, mining consists of excavating the clay and searching through it for the hard crystals of tourmaline and other gem minerals. In some of the dikes, underground mining is necessary. When a mine is in a pegmatite of this kind, the zone in which crystal filled cavities occur must be followed closely and cavities opened very cautiously to prevent blast damage to the gem crystals.

Recently, California has once again become a significant producer of tourmaline. The famous old Himalaya Mine, at Mesa Grande, has been reopened and is beginning to produce some of the magnificent material for which it became noted shortly after the turn of the century. In the early days of the mine, a single enormous cavity produced more than six tons of tourmaline. At the present rate of exploitation it would take many years to equal early production, but many fine specimen quality crystals have been removed, as well as a significant quantity of curable rough. It is financially practicable to mine a pegmatite dikes today only because of the tremendous demand for fine gem mineral crystal specimens. If the mine were operated solely to produce cuttable gem rough, it could not meet expenses. However, when large, top-quality mineral specimens bring in excess of $80 each, the gem quality rough becomes a very profitable by-product.

For many years after its discovery in 1820, the famous Mount Mica deposit, near Paris, Maine, produced some of the world's most beautiful tourmaline. Production during the last three decades, however, has been negligible. In 1954, during prospecting operations for commercial muscovite (mica), a mining company discovered appreciable quantities of gem-quality bluish-green tourmaline crystals near the town of Norway. The gem material is associated with lepidolite, clevelanditei mica, quartz and feldspar. Although current production is sporadic, it is possible that new discoveries will be made with the continuance of mica mining operations.

In recent years two more important sources have come to light in the Indian continent:

  1. India - Tourmalines are found in Pegmatite in a small village called Puttirol in Andhra Pradesh. When the size have gone more than 5000 carats, most of the mining is done in an illegal way, the present Govt. Mining Policy does not encourage the gem mining.
  2. NEPAL has proved to be also a very good source a wide range of quality with color's occur here.

There are a number of other sources of gem tourmaline, significant among which are Madagascar, Ceylon and Burma. Russia and Siberia have also been known as sources. In most of these areas, it is recovered with other gems in alluvial deposits representing the durable portions of both pegmatite dikes and contact metamorphic deposits.

Tourmaline Jewelry

Any style of cutting may be used to fashion tourmaline, including step, brilliant, mixed and cabochon. Also, in recent years a considerable quantity has been carved into flowers, leaves, etc. and is appearing in moderate priced gold jewelry with diamonds and other colored stones.

Dichroism is usually so strong that correct orientation is required to bring out the most attractive colors. Cutting with the table perpendicular to the length of the prism makes the color darker, if not black; if the table is parallel to the length of the prism, the color is lighter. In cutting green tourmaline, other than round stones, the long direction of the table should be made parallel to the axis of single refraction; i.e. parallel to the striated prism faces. The pavilion facets on the ends of dark-green, rectangular, step-cut stones should be very steep, in order to minimize the transmission of the brown dichroic color. Because so much tourmaline is flawed but colorful, it has been used extensively for tumbling.

Facets polish satisfactorily with Linde A powder on a tin lap, and cabochons respond well to either Linde A or chromic oxide on a leather lap. Because the polishing compound often penetrates the hollow tubes in cat's eye material, they must be sealed off. This can be accomplished by heating the stone gently and dipping it in warm liquid shellac. When the stone cools, the shellac will be drawn partly into the tubes. After the polishing is completed, it can be removed by boiling in alcohol.

Because large facets have a tendency to scratch easily, it is frequently necessary to change polishing directions. Large tables polish most effectively if a wood lap is used throughout the operation. A crown angle of 43° and pavilion angle of 39° prove effective.

Physical & Optical Properties of Tourmaline

Physical Properties

Chemical Composition A very complex boron-aluminum silicate, with one or more of the following: magnesium, sodium, lithium, iron, potassium or other metals. Variations of the proportions of these elements cause variations in color. Those containing much iron are dark in color and seldom of earn quality.
Crystallographic Character Hexagonal system. Habit, prismatic crystals, the two ends of which are different in character. Striations parallel to the prism faces of the crystals are common. A cross section is roughly triangular in outline, and is often green on the outside and red or yellow on the inside , or red on one end and green on the other.
Hardness 7 to 7 1/2.
Toughness Fair. Heat-treated green stones are somewhat more fragile than untreated material.
Cleavage None
Fracture Conchoidal.
Specific Gravity Red and pink: 3.01 to 3.06.
Green: 3.04 to 3.11.
Brown: 3.04 to 3.10.
Yellow and orange: 3.10
Blue: 3.05 to 3.11.
Black: 3.11 to 3.20.
Highly flawed material may vary slightly from these figures.
Streak White
Characteristic Inclusions BBoth red and green tourmaline frequently contain irregular threadlike liquid and gas inclusions. They may occur singly or be interlaced with one another in a loose, mesh like pattern. Green tourmaline, especially, may contain dense masses of straight, parallel fibers or hollow tubes that produce a chatoyant effect on cabochon stones. Red material also often contains what appear to be gas filled fractures parallel to the principal axis. Because of their breadth and the fact that they contain gas, they present a mirror like appearance when examined perpendicular to their wide dimension.

Optical Properties

Degree of Transparency Transparent to translucent
Luster Vitreous.
Refractive Index 1.624-1.644, Dark stones may read as high as 1.627-1.657
Birefringence .020.
Optic Character Uniaxial negative
Pleochroism Strong.
Red and pink: red and yellowish red.
Green: blue-green and yellow green to dark brown-green. Blue lighter and darker blue.
Yellowish green: blue-green and yellow-green to brown-green: blue green and yellow green to brown-green.
Alexandrite: brownish red and yellowish green.
Dispersion .017.
Phenomena Chatoyancy and change of color
X-Ray, Fluorescence Pink stones may show a weak violet glow. All others are inert.
Ultraviolet Fluorescence Yellow stones may show a very weak almost indistinguishable, glow; pink stones may show a weak violet glow under both long and short wavelengths.
Absorption Spectra Blue and green: almost complete absorption of the red down to 6400 A.U. in addition to a strong, narrow band in the blue-green at 4980 A.U.
Red and pink: a broad band in the green and lines at 4580 and 4510 A.U. in the blue.

Effects Caused by:

Heat Fuses when heated too strongly. Sudden temperature changes may cause a gem to break.
Acids Not attacked
Irradiation Colorless and pink tourmaline change to a more desirable red under electron bombardment. Although the new color appears to be permanent, the stones develop cracks as they change color, and no method has been found that will prevent such damage, Green tourmaline showed no change as a result of bombardment.

Test and Identification of Tourmaline

Because of its enormous range of colors, tourmaline may be confused in appearance with many other gemstones; however, it is usually readily identifiable by relatively simple methods. Two key factors in its identification are its strong birefringence and dichroism. The strong dichroism is usually visible to the unaided eye when a stone is examined in two directions. The refractive indices are usually near 1.624 and 1.644, with the higher index remaining constant. In this general index range, no other commonly encountered transparent gemstone is as strongly birefringent.

In appearance, tourmaline may be confused with topaz, apatite, glass andalusite, of the gemstones in its usual refractive index range. Topaz is distinguished readily by B.G., since tourmaline floats in methylene iodide and topaz sinks. Glass, of course, is singly refractive. Both apatite and topaz have weak birefringence and show very little doubling under magnification, in contrast to the strong doubling of tourmaline. To distinguish tourmaline from andalusite, a careful R.I. determination is necessary: the high reading for andalusite is not constant and it is considerably below the 1.644 figure for tourmaline. In addition, the absorption lines of andalusite (at 4400 and 4600) A.U.) may sometimes be of help, particularly in testing rough material when an interference figure cannot be obtained.

It is possible to confuse light-green tourmaline with peridot, which, like tourmaline, is strongly birefringent; however, the 1.654-1.690 indices of peridot permit a ready separation. In medium to dark-green stones, the strong dichroism of tourmaline distinguishes it immediately from peridot.

The dark-green synthetic spinel that is sold as "synthetic tourmaline" is identified by its higher R.I. (1.73 ) and S.G (3.60) and its single refraction.

Some spodumene, especially kunzite, may be very similar in appearance to tourmaline. Although their S.G. are close and their R.I. are in the same general position on the refractometer scale, it is not too difficult to distinguish between them on the basis of a careful R.I. determination. Kunzite reads 1.660 to 1.676, but both low and high readings vary as a stone is rotated on the hemisphere, in contrast to the high tourmaline reading, which remains constant near 1.64.

The novice may confuse tourmaline (green and brown, particularly) with glass on the refriactometer because of the orientation of the cut stone optically. The dichroism is so strong that it is advantageous to cut dark stones with the optic axis (direction of single refraction and the direction of almost total absorption) parallel to the table; this is usually also the long axis of the cut stone. When the stone is placed on the refractometer hemisphere in the usual manner, with the long axis parallel to the length of the hemisphere, only a single reading is seen at the high point for tourmaline,. A rotation of 90° is necessary to show the maximum separation between readings. Failure to rotate may lead the novice to the belief that the stone is singly refractive.

Tourmaline may be confused with several varieties of beryl, but only a lack of testing equipment or a very careless use of that available would be likely to cause errors in determinations. Beryl is distinctly lower in both R.I. and S.G. Both dichroism and birefringence make possible a quick separation from emerald.

Valuation of Tourmaline

In medium qualities, tourmaline can be obtained easily in almost any size up to the 15 to 20 carat range; only in the most expensive and very rare colors is it often difficult to obtain.

The green stones sold most commonly are dark in tone. Their hue is usually yellowish green or bluish green; some are slightly brownish. Although some of these colors are improved by heating, the color is permanent and they are desirable as gemstones. Pricing depends on depth and intensity of color; intense, medium to medium-dark tones bring the higher prices.

Although the tendency is to regard the value of any less expensive colored stone on the basis of its approach in appearance to the most expensive gemstone occurring in the same color, they should be judged on the basis of their own beauty. Thus, red tourmaline should be judged on its own merits , for in its finer qualities it does not bear a close resemblance to fine ruby. Most red tourmaline of gem quality is best described as violetish red to purplish red of medium intensity. Since it does not shown the exceedingly strong fluorescence of ruby, the color seems less intense. Often there is a distinct brownish cast to the violetish red; nevertheless, it is a very pleasing stone to most persons. The higher the intensity of color (i.e. , the lower the brownish content) and the purer the red, the more valuable the stone. There is usually a plentiful supply of material of a light or very light tone (pink), and it is inexpensive. These stones have been quite popular in the West.

Pure blue stones are rarely seen; most are dark violetish or greenish blue. Fine blue stones command higher prices than dark violetish blues or greenish blues. Very attractive aquamarine-colored stones (light blue to greenish blue) have brought as much as $50 per carat in the 5 to 15 per carat size range.

Tourmaline cat's eyes of fair to average quality usually cost from $4 to $12 per carat. Top quality stones may bring as much as $15 $20 per carat.

Black and colorless tourmalines find little use in the jewelry trade, and brown stones, unless they are light in color and especially transparent, are of little value.

Alexandrite like tourmaline with a pronounced color change is so rare as to be classed as a collector's item. Greenish hues of a high intensity with the least tendency toward yellow or brown are the most desirable daylight colors, and red with the least content of orange or brown is the most desirable color under artificial light.