98 BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Crystallization. — Isometric; tetrahedral; showing prominent 

 dodecahedral cleavage. 



Color. — Colorless when pure, but usually pale yellow or browD 

 because of the presence of traces of iron and possibly other elements. 



Luster. — Adamantine to resinous; transparent to translucent. 



Hardness. — 4; too soft to be of much use as a precious stone. 



Specific gravity. — 4 ±0.10. 



Optical properties. — Refractive index, 2.37. Isotropic. 



The mineral dissolves in hot hydrochloric acid with evolution of 

 hydrogen sulphide. This and its optical properties will usually 

 suffice for its determination. It occurs in veins, beds, and pockets, 

 and is used chiefly as an ore of zinc. Clear material suitable for 

 cutting as a precious stone is exceptional and its use limited. 



LIST OF SPECIMEN'S. 

 SPAIN (PICOS DE EUROPA. SANTANDER). 



Step-brilliant, square girdle; deep yellow; 12.45 carats; 12 by 9 mm No. 589 



Step-rose, circular girdle; yellow; 6.02 carats; 11 by 6.5 mm No. 590 



SPINEL. 



Varieties. — Balas ruby or spinel ruby. 



Composition. — Magnesium aluminate, MgAl 2 4 , with some iron or 

 other elements replacing magnesium and aluminum. 



Crystallization. — Isometric . 



Color. — Usually red, owing to the presence of small amounts of 

 chromium; also colored green by iron or blue by cobalt. 



Luster. — Vitreous ; transparent. 



Hardness. — 8; an extremely durable stone. 



Specific gravity . — 3 .1 ±0:2. 



Optical properties. — Refractive index, 1.72; in ordinary varieties, 

 optically isotropic. 



With the microspectroscope red and green varieties show no well- 

 defined absorption bands, but blue varieties show a strong band in the 

 green and one in the blue, characteristic of the metal cobalt. The 

 mineral may be distinguished from true ruby and sapphire by the 

 isotropic character, and from other stones by the properties above 

 listed. 



Following the order of the prismatic hues there are red, orange, 

 yellow, green, blue, indigo, and violet colored spinels; and also there 

 are those showing a whole series of intermediate hues, such as pink, 

 heliotrope, lavender, lilac, purple, fawn, corn color, etc. The 

 transparent, lively, red-colored spinel is called spinel ruby, and may 

 readily be taken for the true ruby, though its small refractive and 

 dispersive power, together with the absence of pleochroism, render 

 it less brilliant than and lacking the fire of the red corundums. The 



