88 



formerly much esteemed, and known by the jewellers under 

 the name of Rock Crystals, and Scotch, Welch, or Cornish 

 Diamonds; nor do jewellers seem to distinguish between 

 Rock Crystal and Quartz, although they chiefly use Rock 

 Crystal. 



It is sometimes found yellowish, or of a topaz colour, 

 passing to red, purplish, brown, black, &c. Its lustre is 

 glassy ; it is more or less transparent, and is said by most 

 authors to have a double refraction: we, however, could 

 not discover this circumstance. The fracture is coarse, 

 splintery, conchoidal, or undulating, the flaws frequently 

 iridescent. Hardness 10. Kirw. brittle, strikes fire with steel, 

 and scratches glass. It is the chief ingredient in making 

 glass, when fused with potash, soda, &c. and seems to be 

 only a purer kind of flint. Diamond has generally been 

 classed as the first species of Silex, but it has at length been 

 discovered to be the purest species of Carbon. Quartz 

 seems very properly distinguished from rock crystal by Mr. 

 Kirwan. The former if exposed to a strong red heat be- 

 comes of an opaque white : this specimen is therefore truly 

 quartz, as I have proved by trying a fragment, which being 

 exposed to a strong heat in a common fire became first of an 

 opaque white, and by longer exposure somewhat opaline, 

 or rather like chalcedony ; not unlike common flint under 

 similar circumstances. Rock crystals on the contrary, ori- 

 ginally dark brown, &c. by the same heat become beauti- 

 fully transparent, as some lapidaries and jewellers well 

 know. 



