Mineral Species: Quartz var. Black Opal

Chemical Formula: SiO2

Chemical Name: Silicon Dioxide

Mineral Class: Tectosilicate

Mineral Notes: Opal is a distinct variety of Chalcedony, which is itself a form of microcrystalline Quartz. It forms in fissures and cavities where Silica is precipitated from water over long periods. Once the water flow stops, the material slowly dehydrates leaving behind a mass which gradually mineralises into opal. Even though the material is not crystalline, in the true sense of the word, an order exists; thus opal consists of a dense packing of silica spheres. These cause the refraction of light giving rise to the opalescence which is so characteristic of this mineral.

Description: A mass of black and white Opal on matrix. Although the opalescence is relatively weak, the interest comes in the thick bands of black Opal, which is extremely rare. The colour contrast and the thick, dendritic growths of the black opal makes this a very attractive display specimen.

Location: Bohouskovice, Ceské Budejovice, Bohemia, Czech Republic.