104 



BULLETIN 118, UNITED STATES NATIONAL MUSEUM. 



Color. — Colorless when pure, but often showing disperse colors due 

 to constituents of unknown nature; may be pale blue, pale yellow, 

 or pink; pleochroism weak. 



Luster. — Vitreous ; transparent. 



Hardness. — 8; a very durable stone. 



Specific gravity. — 3. 50 ±0.05. 



Optical properties. — Mean refractive index, 1.62; double refraction 

 weak, 0.008; optically biaxial, positive. 



The best colorless topazes have considerable fire, and, when prop- 

 erly cut, exhibit brilliant reflections of white light, approximating 

 that of the diamond. The pink topaz is probably not known in 



Fig. 12.— Topaz crystal in matrix. 



nature, the delicate tint being commonly obtained by heating the 

 yellow or brown colored stones. The process of "pinking" is quite 

 simple. The selected stone is packed in magnesia, asbestos, or lime, 

 and carefully heated to a low red heat, care being taken that the 

 temperature is raised gradually; the stone is then allowed to cool 

 slowly. If the temperature reached has been sufficiently high, the 

 desired pink tint is obtained; if not high enough, a salmon tint; if 

 too high or too long continued, the color is lost completely. 



There are several distinct minerals which are commonly called 

 topaz — the topaz proper; the yellow sapphire known as the "oriental 

 topaz;" and certain colored quartzes, known as "Saxon," "Scotch," 



