Specimens of opal, a mineral composed of silica, but whose water content is usually between 3 and 9%, and it can even reach 20% depending on the variety. Here many collector’s items are presented with a description, one or more photos and sometimes a video. These opals are for sale.
It’s hydrated silica, with the chemical formula SiO₂ – n H₂O. Opal-AG, the common opal, is formed of microspheres of amorphous silica, associated with water molecules in variable proportions, trigonal quartz crystal system for Opal-AN (hyalite) and for Opal-CT (lussatite) is composed itself of cristobalite-tridymite.
This is not a mineral species in the strict sense of the term, but this family has official recognition by the IMA.