TAB. CXV. 



S I L E X Quartzum. 

 Rock Crystal. 



Class 1. Earths. Order 1. Homogeneous. 



Gen. 4. Silex. Spec. 1. Quartzum. 



Div. 1. Crystallized. 



Rock-crystals have been distinguished by many Mine- 

 ralogists from the manner in which the crystals, depending 

 on the column, swell or thicken in the middle. This 

 mode of discrimination is perhaps tolerably correct, as I 

 believe Rock Crystals alone are thus crystallized. It however 

 does not determine that all Rock Crystals are thus formed, 

 as may be seen by the Cairn Gorm Crystals, tab. 102. 

 We may perhaps say that it can scarcely be otherwise deter- 

 mined from Quartz, unless by fire*; and it has been observed 

 that Rock Crystals rubbed together are phosphorescent, or 

 give light on collision in the dark, and exhale a peculiar 

 empyreumatic odour. All stones as hard as common flint, 

 to the consolidated aerial Diamond, will do this with very 

 little difference. 



Rock Crystal is often accompanied with Chlorite, and of 

 so vivid a vegetable green that it much resembles moss, and 

 has in some instances been taken for such : in this speci- 

 men, parts are impregnated with it, and seem decomposing: 

 the whole has something besides of an opaque whitish cast, 

 rather peculiar to this sort of Crystal. They often have 

 the substance called Lac-Lunae about them. 



* In which Quartz becomes opaque. 



