44 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Cabochon, circular girdle; brown; 5.74 carats; 11 by 7 mm No. 1457 



Cameo; elliptical girdle; brown; 5.025 carats; 14 by 10 by 5 mm No. 1458 



Cabochon, elliptical girdle; brown; 2.025 carats; 12 by 6 by 4 mm No. 1452 



Cabochon, elliptical girdle; two gems, one brown, one green; 1.49 and 1.42 



carats; 9 by 6 by 4, 10 by 5 by 4 mm No. 1458 



Demantoid. — See under Garnet. 



DIAMOND. 



Com/position. — Carbon 

 (isometric form), C. 



Crystallization. — I s o - 

 metric, holohedral ; habit 

 usually octahedral, with 

 rounded edges; cleavage, 

 octahedral (fig. 5) . 



Color. — Co 1 o r 1 e s s 

 when pure; often show- 

 ing nonessential violet, 

 blue, green, yellow, 

 brown, or red colors, 

 owing to the presence of 

 traces of impurities of 

 unknown nature. 



Luster. — Rather dull 

 and greasy in the nat- 

 ural state, brilliantly 

 adamantine when cut; 

 when free from flaws, 

 transparent. 



Hardness. — 10; will 

 scratch every other known mineral; the most durable of precious 

 stones. 



Specific gravity. — 3. 52 ±0.02 noticeably heavy. 

 Optical properties. — Refractive index, 2.42; isotropic in polarized 

 light, or slightly anisotropic if under strain; dispersion of spectrum 

 colors very high. 



Miscellaneous properties. — A rather good conductor of heat so that 

 it feels cold when placed against the skin (in a sensitive spot). 



Occurrence. — Occurs in ultra-basic igneous rocks and in gravels 

 derived by their disintegration. (See Williams collection of rocks 

 from the diamond mines of South Africa, p. 135.) 



Artificial production. — Made by Moissan by the sudden chilling of 

 molten iron containing dissolved carbon; has also been made by other 

 processes involving simultaneous action of high temperature and 

 pressure. 



Fig. 5.— Diamond crystals. Showing characteristic forms. 



(Enlarged 6 diameters.) 



From Gardner F. Williams collection. 



