BLUE AND VIOLET STONES.
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NAME. DENSITY. PLEOCHROISM. HARDNESS.
Lapis-lazuli 2·40 None 5½
Iolite 2·63 Strong 7¼
Amethyst 2·66 Weak 7
Beryl 2·69 Strong 7¾
Turquoise 2·75 None 6
Tourmaline 3·10 Strong
Topaz 3·55 Strong„ 8
Spinel 3·65 None 8
Sapphire 4·00 Strong 9
Zircon 4·65 Weak 7½
On pages 6 to 20 there have been described easy methods of ascertaining the density, pleochroism and hardness of precious stones. The index of refraction may also now be determined, thanks to Mr. G. F. Herbert Smith's improved Refractometer, which is available for all gems of which the index of refraction lies between 1·45 and 1·74. The instrument, as made by Mr. J. H. Steward, is accompanied by instructions how to use it and how to interpret the observations made; also by a table of indices corresponding to each division of the graduated scale. The Refractometer is standardized for use with the yellow light of a sodium flame, but white light may be employed if approximations suffice.
Place a small drop of a highly refractive liquid—α bromonaphthaline, for example—upon the plane surface of the hemispherical lens. Then press a flat face of the gem into the liquid, so that the latter has a uniform thickness. Light being admitted through the ground glass "window" of the instrument, and the scale focussed, it will be seen that different parts of the scale are differently illuminated; the edges or boundaries of these regions where they coincide with the scale-divisions are to be noted. With glass and other singly-refracting substances, the edges due to these solids are single, with doubly-refracting minerals double. It is essential that the edge arising from the refractive liquid itself should be beyond the edge due to the stone. So when spodumene, peridot and spinel, with indices ranging between 1·65 and 1·73, have to be tested, methylene iodide, with a refractive index of 1·74, must be substituted for α bromonaphthaline, the refractive index of which is only 1·658.
