Opal is an amorphous, or non-crystalline form of silica with the formulae SiO2ùnH2O and a relative hardness of 6. It is always a secondary mineral, found lining and filling cavities in igneous and sedimentary rocks where it has been deposited by hot waters. The ordinary varieties are common but the precious varieties are quite valuable. Opal is also a petrifying agent which may permeate fossil wood, corals, shells etc, preserving even the minute structure of the organic remains. It was discovered in 1964 that opal is made up of close-packed silica spheres, and in 1974 Pierre Gilson produced the first synthetic opal.
In folklore, opals are considered unlucky. They are full of 'eyes' which interfere with the sanctity of the privacy of the wearer or home. Research Opal
Abbreviations
Rocks & Minerals
Research Opal