In the following paragraphs a number of the less frequently encountered gem materials are listed whose properties vary sufficiently to warrant short discussion. Those not discussed do not have properly variations that require specific mention.
Actinolite is included in gem tables primarily because nephrite jade is a variety, not because it is likely to be fashioned as a gemstone. However, a knowledge of its properties gives ad idea of the range of possibilities for nephrite. The refractive indices range from 1.614 - 1.641 to maximum of approximately 1.628 - 1.655. Nephrite is likely to be nearer the low set of indices. The S.G. range is 3.00 to 3.20, with a figure of 3.10 most common.
This is a massive variety of gypsum that is used frequently for carved ornamental objects. Since the gemologist may be required to identify materials of this kind, properties are given in most complete tables. The indices of alabaster are usually those for gypsum (1.520 - 1.529). The S.G. of pure material is 2.30, but alabaster has been used for carvings with a density as low as 2.20 and as high as 2.40.
This gem mineral is one in which birefringence is particularly low (.002 to .006). The refractive indices may be as low as 1.628 - 1.632 and as high as 1.647 - 1.651, The S.G. usually is very nearly 3.18 but mainly vary from 3.16 to 3.20.
Typical of the carbonates, the birefringence of azurite is great (.110). The indices are 1.73- 1.84. If the specimen is well polished, a reading may be obtained. Often, the large birefringence may be detected by rotating the stone on the hemisphere of the refractometer. If a Polaroid plate is rotated before the eyepiece, the R.I. will appear to fluctuate between 1.73 and 1.81 (the upper limit of the instrument). Azurite is notable too because of its wide varieties in specific gravity. Although the usual text figure is 3.80, porous may float in methylene iodide.
Gem-quality epidote usually has indices near 1.729 and 1.768, but they may be 1.72 - 1.75. Specimens possessing the lower R.I.'s may have a birefringence below the .045 figures given, or even less than .019. High extremes may reach 1.741 - 1.780. The S.G. varies from approximately 3.32 to 3.48; most gem material is near 3.40.
The R.I. range of kyanite is approximately 1.712 - 1.727 at the low end to 1.720 - 1.735 at the high; gem material usually is midway at 1.716 - 1.731. The S.G. varies from 3.56 to approximately 3.58; gem material has a value of about 3.62.
Although the R.I. range for phenakite is from 1.650-1.666 to 1.680 - 1.696, gem material almost always is within .003 of the low figures. The S.G. range is 2.95 ±.01.
The R.I. figures for prehnite are 1.616 - 1.649; thus, the birefringence is .033. However, since it is a translucent aggregate, rather vague reading usually is encountered; it is near 1.630. The S.G. range is 2.82 to 2.94, with the usual determination near 2.88.
This name is applied to a group of minerals with a rather wide range of properties; in gem material however, the properties are fairly constant. Pink and pale-yellow stones are likely to have indices of 1.550 - 1.572 and a specific gravity near 2.68. Blue scapolite has slightly has slightly lower indices and birefringence, with usual figures of 1.544 - 1.560; its S.G. is near 2.63.
Material cut for gem purposes is translucent blue, green or yellow. As with other carbonates, the birefringence is great (.228), with indices of 1.621-1.849. Rotating on the refractometer hemisphere is likely to produce readings anywhere between the top of the scale and 1.621. Rotating a Polaroid plate before the eyepiece often will help detect this excessive birefringence. The S.G. range is 4.20 to 4.40, the usual figure being very close to 4.30.
Like serpentine, talc is used frequently for carvings, especially by the Chinese. Actually, it is an impure rock with variable properties. An R.I. may be obtained on the refractometer that may range from 1.54 to 1.59. The S.G. is between 2.55 and 2.80. It is the material that has been assigned a hardness of 1 on Mohs' scale; however, in some of its impure forms, the hardness may be greater. When it contains agalmatolite the S.G. is very close to 2.80 and the hardness is 1 to 2½, so it can still be scratched by the fingernail. This latter type is used frequently for carved bookends and similar objects.
Since some gem species are formed under a variety of conditions, their properties rarely are identical for all known deposits. Over a period of time, the variations of each of the important gemstones have become known; however, it is possible that some of those discussed in this assignment may eventually be found in forms that extend their property ranges. When recording changes in the known ranges, the gemologist must be very sure that his determinations are accurate. S.G. determinations, for example, should be accepted only when taken on stones of more than two or three carats in weight by the hydrostatic method, and even then the figure should be rechecked. When using a refractometer to record changes, monochromatic light should be used and, as mentioned previously, the accuracy of the instrument should be checked before a report is made.