Opal Gems and Jewelry
Opal is hydrated amorphous silica, with the chemical formula (SiO2·nH2O). It may contain anywhere from 3% to 21% water by weight. Opal is classified as a mineraloid rather than a mineral. The internal structure causes the gemstone to diffract light, potentially producing a rainbow of colors. Opal is softer than crystal silica, with a hardness of around 5.5 to 6. Opal is amorphous, so it does not have a crystal structure. Citation
