Black Siliceous Onyx 1 Piece Raw Minerals Stones Rocks Collecting (2)

Description

Origin : Brazil

Black Siliceous Onyx 1 Piece 31-50 gr - cm 2.5-5 Raw Minerals Stones Rocks for Collection.
Also available in lots, at this link.

The name Onyx indicates two completely different types of rocks: Siliceous Onyx, normally black in color streaked with white, with a SiO2 composition. n H2O, similar to chalcedony (but not quartz, which is crystalline), found in Brazil, Mexico, and other places; and Calcareous Onyx, also known as "alabastrite pnyx", "Etoca onyx" or "Egyptian onyx", which is composed of CaCO3 (that of Montaione is brown in color or that of Pakistan is light green).
Mineralogically siliceous onyx is a variety of chalcedony, i.e. quartz in compact microcrystalline masses, of an opaque or semi-opaque, uniform colour, which covers red-brown tones and the entire range of grays up to black.
Like all varieties of quartz it is very hard (6 to 7 on the Mohs scale).
It mainly forms in a low temperature and metamorphic vein-hydrothermal environment, or, secondarily, in sedimentary rocks where it occurs in a massive and stratified form taking the name of flint, a material much used by man in prehistory and antiquity for the preparation of sharp objects and jewelry.