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PRECIOUS STONES
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Hardness. Specific Gravity.
Sapphire...............................9 3.9 to 4.1
Topaz.................................8 3.50 to 3.56
Euclase................................7.5 3.05 to 3.1
Aquamarine...........................7.75 2.68 to 2.75
13. Fluorite and other Sea-Green Stones
Fluorite is so much softer than other stones of this color, that it is easily known (hardness, 4).
14. Siam Ruby and Almandine
The
purplish red of these two stones is sometimes similar, but the harder
lustre and sharper transparency of the ruby are generally unmistakable.
If there is doubt, the double refraction of the ruby will distinguish it.
15. Purple Spinel and Sapphire
These
are often so much alike that it becomes necessary to have recourse to
the dichroiscope as the easiest method of deciding. The spinel is
single refraction, the sapphire double.
16. Amethyst, Siam Ruby, Almandine, Spinel, and Sapphire
The amethyst is more decidedly purple than the Siam ruby or the almandine, which are red with purple tints, and is a redder purple than the purple spinel and sapphire, which are darker and more inky.
17. Axinite, Apatite, and Fluorite (Purple) These
are seldom seen. Axinite shows a decided leaning to brown, and is
strongly dichroic (violet, brown, green) ; the twin colors of apatite
are weak, and fluorite is singly refractive.
Hardness. Specific Gravity.
Axinite.................................6.75 3.29 to 3.3
Apatite.................................5 3.2
Fluorite.................................4 3.1 to 3.2
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