Occasionally clear colourless felspar has been faceted, and then closely resembles rock-crystal. A careful determination of the refractive indices and the specific gravity serves to discriminate between them. [Illustration: _PLATE XXIX_ 1. CAT’S EYE 2. PERIDOT 3. PERIDOT 4. MOONSTONE 5. HESSONITE 6. PYROPE 7. DEMANTOID 8. ALMANDINE 9. SPODUMENE 10. KUNZITE 11. HIDDENITE 12. ZIRCON 13. ZIRCON 14. ZIRCON 15. ANDALUSITE 16. NEPHRITE 17. TURQUOISE 18. JADEITE GEM-STONES] CHAPTER XXXII TURQUOISE, ODONTOLITE, VARISCITE Of all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a prominent place in jewellery and can aspire to rank with the precious stones. The colour varies from a sky-blue or a greenish blue to a yellowish green or apple-green. Only the former tints, which are at the same time the rarer, are in general demand, and they possess the great advantage of harmonizing with the tint of the gold setting. The blue colours are, especially in the case of the Siberian stones, by no means permanent, and fade in course of time. Turquoise is amorphous and seldom crystalline, and is therefore somewhat porous; it should consequently never be immersed in liquids or be contaminated with greasy and dirty matter lest the dreaded change of colour be brought about. The stones are translucent in thin sections, and a good observation is possible with the refractometer if the back of the stone is flat and polished, since only the section immediately adjacent to the instrument is concerned; the refractive index is about 1·61. The specific gravity varies from 2·75 to 2·89. Turquoise has a hardness of slightly under 6 on Mohs’s scale, and takes a good polish, which is fairly durable, since on account of the comparative opacity of the stones scratches on the surface are not very noticeable. In composition it is a complex phosphate of aluminium and copper, corresponding to the formula CuOH.[6Al(OH)_{2}].H{5}.(PO_{4})_{4}, with ferric oxide replacing some alumina. The blue colour is due to the copper constituent, and the predominance of iron may cause the greenish shades; but the water contained in the stones plays no mean part, since they turn a dirty green when it is driven off. The faded colour can sometimes be restored by immersion of the stone in ammonia and subsequent application of grease, but the effect is not lasting. Attempts are sometimes made to improve inferior stones by impregnating them with Berlin blue, but with only qualified success. Turquoises are said to be affected by the perspiration from the skin.