Opal is a hydrated amorphous form of silica. The internal structure of precious opal causes it to diffract light, resulting in play-of-color. Depending on the conditions in which it formed, opal may be transparent, translucent or opaque and the background color may be white, black or nearly any color of the visual spectrum. Opal is rated between 5.5 and 6 on the Mohs hardness scale.  Opal is thought to increase emotional activity and awaken the sexual center in the body.