Even when testing gemstones of the same species, the gemologist may encounter distinct differences in important properties from one stone to the next. This may prove disturbing and lead him to question that they belong to the some species. In this assignment, the species and groups in which variations are likely to be noted are discussed.
Property variations in some species and groups are rather wide, but in others they are almost nonexistent. Varieties of quartz in which a stone is cut from a single crystal (e.g., amethyst and citrine) are known for the consistency of key properties. The specific gravity of single-crystal quartz may be depended on as just slightly over 2.65 and the refractive index as 1.544-1.553. Any variation beyond .002 from these figures should cause the alert gemologist to question either the accuracy of the means used to determine the values for those properties or to question the identity of the gem.
Questioning property variations has led to some interesting discoveries. For example, a gem mineral of fairly recent discovery was particularly interesting because it was first noted not in the rough but as a fashioned gemstone. Peridot is an intermediate member of what is known to mineralogists as the series, or group, of minerals of which forsterite (magnesium silicate) and fayalite (iron silicate) are the end members. One would expect, as the iron or magnesium content were increased with respect to the other, that the properties would change markedly, since the R.I. and S.G. of the end members are considerably apart. However, green peridot has always shown indices close to 1.654 and 1.690 and a specific gravity range of 3.20 to 3.37. So-called brown peridot, on the other hand, has had indices close to 1.665 and 1.705 and a specific gravity near 3.48. The green material, in addition, always had a positive optic sign, whereas the brown frequently was found to be negative. Again, since some members of the forsterite-fayalite series were positive and others negative, this did not cause too much concern. However, George Switzer, Ph.D. of the Smithsonian Institution, began to question the identity of brown peridot when he found that its optic angle (i.e., the angle between the two axes of a biaxial mineral) was much smaller than seemed expectable in the group. Subsequently, he proved by X-ray diffraction study of powder scraped from the girdle of a gem previously identified as brown peridot that it was an entirely different mineral. This material has since been described in England and named sinhalite. It was found to be an iron-magnesium borate containing no silicon. Thus, what had been considered for more than fifty years as a property variation of brown peridot proved to be a characteristic of an entirely different mineral.
If gem tables, property variations are shown by one of two methods. If the normally encountered R.I. lies midway between the possible extremes, it is listed and followed by a plus-minus figure; e.g., almandite, 1.790 (±.03). If the most commonly encountered index occur closer to one of the possible extremes than the other, it is listed and followed by separate plus-and-minus figures; e.g., opal, 1.45 (-.08, +.02). This means that the R.I. for opal may be as low as 1.37 or high as 1.47, but the most common figure is 1.45. If material suspected of being opal shows figures above or below the range given, it is possible that: (1) it is not opal, ;2) the refractometer is inaccurate or has been read incorrectly, or (3) opal with an R.I. not previously encountered has been found. To check a refractometer in this general range of indices is a simple matter, since rock crystal earl be depended on to show indices of 1.544 and 1.553. If there is reason to believe that the instrument is inaccurate, it can be checked easily with rock crystal for the low end of the scale and synthetic corundum (1.762 -1.770) for the high.
PROPERTY VARIATIONS OF THE MORE COMMON GEMSTONES
Amber
The R.I. of amber usually is very near 1.54, with range extending from approximately 1.539 to as high as 1.545. The S.G. range of flawless material is about 1.05 to 1.096; cloudy material containing many bubbles may be as low as 1.0. Gem-quality material usually is near 1.08. Since a saturated salt solution has a density of about 1.13, all amber should float in it.
Andalusite
The property range of andalusite is not great. However, since it varies considerably in birefringence, it is listed among the gems of notable property variations. The usual indices are approximately 1.634 - 1.643, with a birefringence of .009; but when they are at the low extreme, the birefringence is high. Thus, if the indices are 1.627 - 1.640, the birefringence is .013. When they reach the high extreme (1.639 - 1.648), the birefringence is reduced to only .009. The S.G. varies from 3.13 to 3.21 with a normal figure of 3.17 for transparent material.
Beryl
Rather property variations exist in the beryl species, usually depending on color. In emerald, however, wide variations exist that seem to be characteristic of the deposits from which they are mined. Of the variations in beryl other than emerald, low-property values are encountered in the yellow and golden types and high properties in the red to violet varieties.
Synthetic Emerald
The properties of most synthetic emerald are slightly lower then the usual low figures for natural emerald. Whereas the S.G. range of the natural is approximately 2.69 to 2.74, the usual figure for the synthetic is 2.66. However, Linde hydrothermal and some Gilsons may be 2.685, which is in the same range the natural. The indices of these two synthetics may also be in the range of most natural emerald. Birefringence is also in the range of natural emerald; i.e., .006 -009. Indices for Chatham emerald are 1.561 and 1.564 with a birefringence of .003. Rarely, natural material from Colombia may give readings as low as 1.566 and 1.570. Lower figures have been shown in some writings but have rarely been encountered.
With the exception of iron-rich Gilsons, most synthetic emerald fluoresces to UV. Most natural emerald dose not fluoresce.
Calcite
A very common calcium carbonate mineral having a nearly constant R.I. of 1.486 - 1.658 and birefringence of .172. The specific gravity is near 2.71 end hardness is 3. It effervesces strongly to hydrochloric (muriatic) acid. Single crystal calcite possesses very easy three-directional cleavage, not apparent in the crystal-line aggregate forms which may be used for ornamental purposes or dyed to imitate coral or other materials. The excessive birefringence in crystalline aggregate materials may be apparent if the "spot method" of refractive index determination is employed. It may be necessary to try different orientations of the stone and to rotate a Polaroid plate before the refractometer eyepiece to obtain results.
Property tables show a possible variation of ±; however, higher indices, up to 1.76-1.77, have been encountered. The S.G. is very near 3.73, and the extremes vary not more than .02 from this figure. Chrysoberyl has a small angle between its two optic axes and the intermediate index is very close to the low figure, with seldom more than a .001 difference between the middle and low index.
Coral
Since coral is composed largely of minute calcium carbonate crystals, the R.I. range is the same as that of calcite, in which form the calcium carbonate is crystallized; thus, indices from 1.406 to 1.658 are possible. Since calcite is negative in sign, the high reading is constant. Sometimes, by the rotation of a Polaroid plate in front of a spot reading taken on a curved surface, a plot that is light in the 1.50 to 1.60 range will darken as the plate is rotated. The S.G. range is 2.6 to 2.7 and the hardness is 32 to 4.
Black coral from Hawaiian waters has an R.I. of approximately 1.56- 1.57, with a birefringence of .01 and a specific gravity of about 1.37. It displays no fluorescence under ultraviolet radiation or X-rays. Very thin sections are reddish when held over a strong light source. It is slightly sectile and could possible be confused with plastic, if this test alone were used. Magnification of a cross section reveals a radial structural pattern, which is typical of true coral. Unlike true coral, however, it is not attacked by acids. These properties are not consistent with those of the usual pink coral seen in the trade. This is because black coral is almost pure conchiolin, in contrast to the predominance of calcium carbonate in the pink material.
Golden coral, also from Hawaiian waters, has an R.I. in the same area as the black but the S.G. is somewhat higher, being nearer to 1.70. Some of this material has been found as high as 2.12. Like the black coral, the golden is also almost pure conchiolin. The hardness for both black and golden coral is under 3.
Corundum
This is another gemstone in which variations from standard figures are not great. The lower index may vary from 1.759 to 1.770 and the higher from 1.767 to 1.778. Usual figures are 1.762 and 1.770, with the slightly higher readings expected in dark Siamese ruby and green sapphire. The S.G. of gem material is 4.00 ±.03. The R.I. range is +.008, -.003.
Synthetic Corundum
The property values of synthetic corundum are very close to those of the natural material; namely, 1.762 and 1.770 for the indices and 4.00 for the S.C.
Diamond
Property variations in diamond are very small. Since the R.I. is too high for determination by ordinary means, only the S.G. Is likely to be measured by gemologist. A variation of more than .01 from the usual figure of 3.52 is uncommon.
Diopside
This mineral, a member of the pyroxene group, has a rather wide range of property values. Low indices are 1.665 - 1.69 and the high figures are approximately 1.704 - 1.730, the usual gem material reads at approximately 1.675 - 1.701. The S.G. range is about 3.26 to 3.32, with a usual figure near 3.29.
Moonstone
Except for the most important variety, moonstone, property variations within the feldspar group are not ordinarily considered. Adularescence is possible not only in orthoclase but also in almost any member of the plagioclase series. Thus, although the usual moonstone indices are those of orthoclase (1.518 - 1.526), indices characteristic of albite or oligoclose are not uncommon, but even those characteristic of labradorite have been encountered. This would put the upper limit for gemstones showing a moonstone effect at 1.559 to 1.568, but these are rare. It is not unusual however, to encounter a 1.54 index in a moonstone. Since quartz varieties do not show the blue adulrescence of moonstone, the plagioclase variety should not cause difficulty. Semi-translucent gray labradorite, with its iridescent color effects, is less likely to show wide variations. The indices for labradorite are 1.559 - 1.568, and the S .G. is 2.70 (±.05). The amazonite variety of microcline shows little property variation; indices of 1.522 and 1.530 are usual. The S.G. may be considered constant at 2.56(±.01).
Fluorite
The R.I. of fluorite or fluorspar is very close to 1.434 and the specific gravity is relatively constant at 3.18 (±.01). It has a hardness of 4. Fluorite occurs in the cubic system and has easy, perfect cleavage.
Garnet Group
If every member of the garnet group occurred in a pure form, property variations would not be wide; however, a pure single species is almost unknown, since they are mixed together in almost a complete series of proportions. Thus, to distinguish between two species on the basis of properties is difficult. In a somewhat arbitrary method used to distinguish between members of the group, a combination of properties and color is used in assigning a species name to a garnet. There are theoretical indices of pure garnets of each species; almandite, 1.83; pyrope, 1.705; andradite, 1.885; grossularite, 1.735; spessartite, 1.815. However, every garnet is likely to have significant percentages of several metallic ions; therefore, pure pyrope, almandite, etc., are not encountered in nature. The garnet known as rhodolite is a mixture of almandite and pyrope in nearly equal parts. For the purpose of testing, use the following system: red to brownish-red garnets above 1.759 in index are called almandite; those below are called pyrope. In the range 1.750 to 1.770, those that are more transparent than the brownish-red garnets and that tend toward a purple-red to amethystine color are called rhodolite. Purple garnets above 1.770 are rare but are called almandite in this system. Any brown to orange to red-orange stone with an index of 1.80 to 1.82 is called spessartite. The green translucent grossularite usually is 1.720 in index. The transparent green and colorless grossularite and hessonite will usually show an index of 1.740. Pyrope with an index below 1,735 is very rare (for the almandite molecule is certain to be present); generally, however, pyrope shows a reading very close to 1.746, although stones with an index as low as 1.72 have been encountered. The S.G. for pyrope is given as 3.78 (-.16. +.09).
Glass (man-made)
The extreme index range for glass is from 1.44 to 1.90. The normal range for glass used for jewelry purposes is 1.48- 1.70. The specific gravity can range from 2.30 to 4.50. The hardness will vary depending on the materials used in the manufacture of the glass.
Hematite
Since the indices of hematite are far above the limits of the refractometer, variations are not measurable by means available to the gemologist. The S.G. usually is close to 5.20(11.08), but rarely is somewhat less, with a low figure at 5.12, which is in the range of the latest type of sintered substitute, Hemetine.
Idocrase / Vesuvianite
The form of this mineral used most frequently for gem purposes is the massive green material that resembles jade. Its S.G. is very close to that of jadeite, usually barely floating or sinking very slowly in methylene iodide; the general range is about 3.30 to 3.50. The R.I.'s of the two minerals, however, are sufficiently different to afford easy discrimination. It is interesting to note that within the R.I. range for idocrase a change occurs in sign from negative at one end to positive at the other; in the usual range it is negative, with indices near 1.705 - 1.713. Positive material may have indices of 1.716 and 1.721.
Iolite
Although this mineral has a very wide range of refractive indices, in gem material it is not great; viz. as low as 1.531 - 1.540 or as high as 1.552 - 1.562. Non gem-quality material may read as high as 1.587 and 1.596. The most common figures are 1.542-1.551. The S.G. usually is near 2.62, but a range of 2.56 to 2.66 has been encountered.
Ivory
The properties given here are for elephant ivory since that is the material most often encountered in the jewelry trade. The R.I. is 1.54, the S.G. is 1.85 (±.15). Its hardness is about 2 ½. Variations outside of these properties may be found, depending on the source of the material.
Jadeite
A broad, hazy R.I. reading near 1.65 or 1.66 is shown by jadeite; therefore, variations are difficult to see. The S.G. is usually just above that of methylene iodide (3.32), at 3.34, but may vary by ±.04. Some jadeite specimens sink very slowly in this liquid, whereas others barely float.
Jet
The index range of jet is from 1.64 to 1.68. A single clear reading is possible on well-polished material. The S.G. may vary from as low as 1.10 to as high as 1.40 but the extreme readings are unlikely for high-quality material. The range for fine jet is approximately 1.30 to 1.34.
Lapis-lazuli
Like jadeite, the index of lapis is visible only as a broad band; it is usually close to 1.50. The S.G. is one of the most variable of the important gemstones. Although it usually is near 2.75, material without visible pyrite may have an S.G. as low as 2.50, but material with considerable pyrite may reach 3.00 or even as high as 3.10.
Malachite
Because it is deposited in finely crystalline form in fairly cool solutions, malachite often is quite porous; therefore, the S.G. variation is very wide. In most textbooks, the figure is given as 3.95, but it is not uncommon to encounter material of such high porosity that it floats in methylene iodide. Compact material may reach the 3.95 figure. The R.I. usually is difficult to obtain precisely. Since the mineral is a carbonate, the birefringence is very high. The indices for pure material are 1.66 and 1.91. On the refractometer, a broad band may be encountered in the vicinity of 1.66. If a Polaroid plate is rotated before the eyepiece of the instrument, the reading varies from the 1.66 to a position in which the scale becomes dark all the way to the liquid, at 1.81.
Moldavite
The properties for moldavite are only slightly variable. The R.I. is usually near 1.48 but may be as high as 1.52. The specific gravity is 2.40 (± .04) and the hardness is about 5.5.
Nephrite
As it is with jadeite, the R.I. reading for nephrite usually is a broad band in white light and difficult to get in monochromatic light; therefore, the variation, unless very marked, is unlikely to be noted. The usual index is near 1.61, but extremes of 1.606 to 1.632 may be encountered. The S.G. is close to 2.95, although it is variable between 2.90 and about 3.00.
Obsidian
As with moldavite, the properties of obsidian are relatively stable. The R.I. being 1.50 (±.02) with the reading tending toward the lower range. The S.G. is 2.45 (± .10) and its hardness is 5 to 5 ½.
Opal
The R.I. of opal varies widely. Most gem-quality material with a play of color is near 1.45 (±.01). Fire opal may read as low as 1.37, but usually it is between 1.38 and 1.42. The usual S.G. near 2.15, but it may vary from 1.25 to 2.22
Pearl and Mother-of-Pearl (Shell)
Since pearl is composed largely of calcium carbonate in the form of aragonite, refractive-index possibilities range from 1.53 to 1.686. And because aragonite is negative in sign, high readings are always present. The usual S.G. of natural pearls is 2.70, although extremes of 2.68 and 2.85 may be encountered; some Australian types have a 2.78 S.G. Cultured pearls usually are 2.70 to 2.78, with the highest percentage near 2.74 to 2.75, slightly higher than the average natural pearl. Generally, freshwater pearls are slightly higher in S.G. than the salt-water variety, but slightly lower than the Japanese cultured product. Usually, mother-of-pearl is 2.70 to 2.90. Both natural and cultured pearls of irregular shapes may have rather large empty spaces and correspondingly lower S.G.'s.
Plastic
The property variations for plastic are rather wide since the types of plastic are many. The R.I. may range from 1.46 to 1.7. The S.G. is 1.30 (± .25) and the hardness ranges from 1 ½ to 3.
Peridot
Although peridot represents an intermediate point between end members of a group, the properties of the green variety are remarkably constant. Refractive indices usually are within .004 of 1.654 and 1.690. The S.G. range is 3.31 to 3.48; thus, it usually sinks slowly in methylene iodide. Peridot is biaxial and the intermediate index usually is within .001 to .002 of the midpoint between the highest and lowest indices. The angle between the two optic axes always is near 90°. Brown peridot, which is much less commonly seen than the green variety, has properties similar to those of the green, with the beta index near the midpoint.
Quartz and Chalcedony
Single-crystal quartz varieties, as mentioned previously, are remarkably constant in their properties. The indices always are within .002 of 1.544 and 1.553; and the S.G. is just over 2.65. Finely crystalline quartz, as distinguished from both cryptocrystalline and single-crystal types, has indices very close to those given for single-crystal quartz, but the S.G. may be as much as .02 away from the 2.65 figure. Chalcedony is lower in S.G. and R.I. than the crystalline material. Its S.G. may be given as 2.60 (±.05), and its R. I. usually is between 1.535 and 1.540 although rarely it may be as low as 1.530. In monochromatic light, two readings often are seen, usually at about 1.535 and 1.539; in white light, they are rather broad and indistinct.
Rhodochrosite
This manganese-carbonate member of the calcite mineral group occurs both in single crystal and crystalline aggregate from. It has rhombohedral cleavage; indices of 1.597 and 1.817; as hardness of 3 ½ to 4 ½. The S.G. for single crystal material is about 3.70 and slightly lower for the massive, agate-like material.
Rhodonite
When pure, rhodonite has refractive indices of 1.730-1.740; impurities are likely to reduce the R.I. to about 1.720 - 1.730. It may be either positive or negative in optic sing. The S.G. range is rather great (approximately 3.30 to 3.70), depending on the amount of impurities.
Serpentine
In various forms, serpentine is usually carved as ornamental objects or used as a jade substitute. Because of very weak birefringence, only one index can be obtained on the refractometer; depending on the specimen, the reading may vary from 1.56 to 1.57. The S.G. range for most serpentine is 2.51 to 2.63, and the hardness is 2 to 4. Bowenite, the most gemlike variety, has a hardness of 5 to 6 and an S.G. of 2.60 to 2.80.
Sodalite
This mineral crystallizes in the cubic system and has no easy cleavage. It has a refractive index of 1.483 with a possible variation of ±.003. The specific gravity is usually 2.24 (±.05) but may go as high as 2.35. Its hardness is 5 to 6.
Spinel
Although it is seldom referred to as such by gemologists, spinel is considered by mineralogists to be a member of a large group of minerals. It is a double oxide of magnesium and aluminum.Since many other double oxides also occur in the cubic system they are considered members of the spinal group. Although gem spinel almost always has an R.I. near 1.718 and S.C. near 3.60 some dark-blue, green and black specimens may have an index as high as 1.78 and an S.G. as high as 4.0. An index higher than 1.72 is uncommon. The hardness is given as 8, but gem material at or below 7 has been noted.
Synthetic Spinel
The property values of synthetic spinel are appreciably higher than those normally encountered in similar colors of the natural material. Very rarely, however, natural dark-blue or dark-green spinel is encountered with property values that exceed those of the synthetic. The R.I. range of the manmade stone is 1.720 to a high of 1.740. The usual reading, however, is 1.730. The S.C. is about 3.64 in contrast to the most common figure of 3.60 for the natural.
Spodumene
This mineral has refractive indices of 1.660 - 1.676, with a possible variation of ±.005. The S.G. usually is very near 3.18 although variations of up to .03 have been noted. The optic angle of spodumene, which is always biaxial positive, is fairly small. Usually, the beta index is near .005 to .006 above the lowest.
Strontium Titanate
The R.I. of this manmade product is 2.409 and the S.G. is within ±.02 of 5.13.
Synthetic Rutile
Little variation in properties is shown by synthetic rutile.
The S.G. ranges from 4.24 to 4.28 (average 4.26), and the usual
refractive indices are 2.616 -2.903, with a consequent birefringence
of .287.
Topaz
The properties of topaz vary distinctly with color. Yellow to brown or light-red stones have higher indices, usually near 1.629 and 1.637. Colorless and light-blue material, on the other hand, have indices near 1.609 and 1.617. The birefringence in each case is about .008. The S.G. for the yellow to brown and red stones is 3.52 (± .02); for the colorless, blue and green, 3.56 (±.03).
Tourmaline
Tourmaline is a very complex gemstone and is regarded by many mineralogists as a group, rather than a single species. For gemological purposes, it may be regarded as a species in which certain property variations occur as a result of a change in coloring matter. The R.I. variations are not great within the normal gem range, being 1.624 - 1.644 (+ .006), with a birefringence usually near .020 but varying from .018 to .020. Very dark to black stones may give both higher readings and higher birefringence. The S.G. may be stated generally as 3.06 (± .05), but light-red stones are on the low end of this range and blue on the upper end. Black tourmaline may reach 3.21, or higher.
Turquoise
The refractive indices of turquoise are given as 1.61 and 1.65;
however, in most refractometer readings in white light, only a
single index is noted, usually between 1.60 and 1.61. The S.G is
2.76, with a variation of -.45 +.08. Chalky material may be 2.3 to
2.4. A figure below 2.6 in material that is not chalky suggests
treatment.
Y.A.G.
This material is over the limits of the refractometer with an R.I. of 1.833. The S.G. is near 4.55 with the green material going as high as 4.57. The hardness is 8¼.
Zircon
This mineral is particularly noted for its wide range of property values. This seems to be accounted for by a breakdown in structure, which may be caused by the presence of small amounts of the radioactive element hafnium. Over a period of time, zircon breaks down from its normal tetragonal structure to monoclinic zirconium oxide and glass. Zircon is found in nature in three stages.
The original crystal structure is intact or nearly so. The indices are near 1.925 and 1.984, and the S.G. is within .03 of 4.70. In this high-property stage, zircon may be yellow brown or colorless, as it sis found in nature; after heat treatment it may be colorless or blue.
The second type is called medium-property zircon; it represents a state between the high and low types, in which the crystal structure has broken down partially. Heat treatment usually changes it to high-property zircon, apparently by restoring its original structure. Medium-property material usually is yellow-green to green, but it also may be red to greenish brown. Its properties usually are nearer those of high-property zircon than of the low property material; red zircon, for example, may have a specific gravity near 4.45 to 4.50. In general, the birefringence of medium zircon is, although slightly less than that of the high-property type, still large.
Low-property zircon is usually green, but rarely it may be orange or brown. Its refractive indices are in the low 1.80's and the birefringence is small (near .005). Rarely, its R.I. is low enough to give a faint blue line at the top of the refractometer scale in white light. The usual S.G. is from 3.93 to 4.07.
Zoisite
The blue transparent variety of zoisite has indices of 1.691 - 1.704, specific gravity of 3.30 ±.10) and hardness of 6 ½ to 7. The hardness of the translucent green massive variety and the light to rose-red variety is slightly less at 6 to 6½.
