Fire Opal - Mexican Sterling Silver PendantOrange as volcanic glass, from the same volcanic state that produced it. Fire opal is a variety of opalhydrated amorphous silica (SiOnHO)distinguished by its transparent to translucent orange to red body color, which comes from iron oxide trace content rather than the play of color associated with precious opal. Mohs hardness is 5. 56. 5. The primary and most important source of fire opal in the world is the state of Quertaro, Mexico, where volcanic