Opal

Opal is a variety of quartz, often used in jewelry. Its color can be changed by the heat of the palm of the hand. It is a hardened silica gel that contains up to 10% water. Due to their disorganized structure, opals do not have crystalline forms to keep themselves whole, and can dry out and break. When exposed to ultraviolet light, it often emits a yellow or green fluorescence. It is insoluble in acid and if heated, it loses its water and decomposes, being able to convert into chalcedony or quartz.