498 



REPORT OF NATIONAL MUSEUM, 1900. 



cut ('{i])ochoii with the .summits cutting- the vertical axis of the prism, 

 is due to numerous minute crystals or layers within the stone which 

 reflect the incident light so as to produce the stellar efi'ect. These 

 rays are invariably white, though the specimen may be of any color, 

 and may be best seen by artificial light. This chatoyanc}', when 

 marked, gives the asteria, or star stone, also known as the star ruby or 



Fig. 6. 

 CORUNDUM CRYSTALS. 



Ceylon. 



Specimen No. 81«1, U.S.N.M. 



star sapphire, as the case may be. Should the gem assume a tibrous 

 texture, the chato3"ancy affords the " cat's-eye " ray. 



Corundum is associated with crystalline rocks as granular limestone 

 or dolomite, gneiss, granite, mica slate, and chlorite slate. The finest 

 sapphires are usually obtained from the IxhIs of rivers, either in modi- 

 fied h(\\ag()ii!d i)risms or in roll(>d mass(>s, accompanied by grains of 

 magnetic iron ore, and several species of gems. 



