Fire opal from Mexico . Chemical formula SiO 2 . H 2 O Crystal system Amorph. Cleavage None, conchoidal fracture. Sometimes bluish black or red.
Precious varietys have a rainbow colorplay.
Luster Vitreous to greasy. Commonly around hot springs. Often fluorescent in UV light. Insoluble in acids. Decomposes when heated, giving off water, and may turn into quartz.