An amorphous (as opposed to crystalline) gemstone that displays spectral colours known as "fire" on a white, black or grey background.
The opal family comprises three groups: the "noble" iridescent opal, the fire opal and the common opal.
The orange fire opal is mined in Mexico and is generally uniform, without colour play. Because opals have a high water content, they can dry and crack if kept in a too hot, dry place. These properties, and a hardness of just 5.5-6.5 make the opal a fragile stone. It has a specific gravity of 1.98-2.50.