258 DESCRIPTIVE MINERALOGY. 



conchoidal, even, uneven and splintery. Lustre splendent and vitreous in the crystallized 

 specimens ; sometimes inclining to resinous, and sometimes only shining or glimmering. 

 Varies from transparent to opaque. Brittle and easily frangible. Hardness 7.0. Specific 

 gravity from 2.50 to 2.70. Before the blowpipe alone it is infusible ; but with carbonate of 

 soda, it fuses with brisk effervescence into a transparent glass. 



Varieties. Rock Crystal. This includes the transparent and highly crystallized varieties. 

 It sometimes presents beautiful iridescences, both superficially and internally. Sometimes 

 also it encloses foreign substances, as anthracite, bitumen, titanite, oxide of iron, etc. 



Amethyst. This name is applied to the pyramidal translucent and transparent varieties, 

 which have a violet blue tint. It becomes white and opalescent by long exposure to heat. 



Milk Quartz and Rose Quartz. These occur massive, and are only distinguished by their 

 colour ; the former presents a milky aspect, while the latter has frequently a fine rose-red 

 colour, which is supposed to be derived from a minute admixture of manganese. 



Common Quartz. This includes those varieties which are of a grey or white colour, with 

 a low degree of transluccncy, and occur massive and disseminated. 



Prase. This has a leek-gi-een colour, a resino-vitreous lustre, and a conchoido-splintery 

 fracture. 



Yellow and Brown Quartz. These are of various shades of yellow and brown, and more 

 or less translucent. or transparent. 



Iron Flint or Ferruginous Quartz, presents various shades of yellow and red, and occurs 

 both massive and crystallized. It is opaque. 



Radiated Quartz, occurs in crystals which are closely aggregated, and which radiate from 

 a point. 



Fibrous Quartz. This is produced when tlie composition presents thin columnar particles. 

 The Cat's-eye is a variety of this, interspersed with thin filaments of asbestus, which, when 

 the stone is cut, present a peculiar opalescent appearance. It is usually of a greyish or green- 

 ish colour, but sometimes brown or red. 



Hornstone, includes those varieties with dull colours, which occur massive and in extrane- 

 ous external shapes, with a splintery or conchoidal fracture, dull and glimmering lustre, and 

 which are opaque or translucent only on the edges. 



Hyalite — Muller''s Glass. This occurs in white and transparent botryoidal masses, or in 

 stalactites. It has a vitreous lustre, is brittle, but is as hard as quartz. 



Flint. This has a grey, brown or black colour ; generally occurs massive, and in various 

 imitative forms ; has a glimmering lustre, conchoidal fracture and feeble transluccncy. 



Calcedony. The semi-transparent and translucent varieties, in massive and various particu- 

 lar external forms, with an even and dull fracture. 



Chrysoprase. This includes the beautiful apple-green, strongly translucent varieties. 



Carnelian. The semi-transparent and strongly translucent varieties, with glistening or 

 shining vitreous lustre, and with various tints of red, brown, yellow, green and white. 



Jasper. Those varieties in which the colours are red, brown and black, and sometimes 



