Opal (2) Raw Minerals Stones Rocks Collecting

Description

Origin : Brazil

Opal 20-30 gr - cm 3-4.5 Raw Minerals Stones Rocks for Collection.

Opal is an amorphous colloidal mineral, consisting of hydrated silica (SiO2 nH2O), semiamorphous or microcrystalline, i.e. without a reticular structure, in fact it does not appear in crystals, but in veins, nodules and crusts of various colors, often with iridescence . The color varies from transparent to milky white, with an infinite number of different intermediates (green, red, yellow, brown, black). The water content can reach up to 20%. The opal is formed by slow geological deposit of a low temperature colloidal silica gel.
It includes many varieties including common opal, noble opal, black opal, water opal, fire opal, xyloid (fossil) opal, and hyalite.
The word "opal" has a common root in Sanskrit, Greek and Latin with the meaning of "precious stone".
97% of the world's opal production is in Australia, a country that has made it its national stone. Important deposits are also located in Mexico and Ethiopia.
The opalescence, the play of colors and light presented by the opal samples, is due to interference effects and to the diffraction of light caused in turn by the regular arrangement of the silica spheres, which are arranged in a packed, regular shape and three-dimensional; it is therefore similar to the arrangement of crystals.
Heating opal samples can cause them to dehydrate and therefore lose their opalescent effect.
In Italy it can be found in Sardinia at Sant'Antioco.