Rock Crystal

Rock-crystals have been distinguished by many Mineralogists from the manner in which the crystals, depending on the column, swell or thicken in the middle. This mode of discrimination is perhaps a 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 determined 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 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 specimen, parts are impregnated with it, and they 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-Lunæ about them.