Opal is a hydrated amorphous form of silica; its water content may range from 3 to 21% by weight but is usually between 6 and 10%. Because of its amorphous character, it is classed as a mineraloid, unlike crystalline forms of silica, which are classed as minerals.

Opal has always been associated with love and passion, as well as desire and eroticism. It is a seductive stone that intensifies emotional states and releases inhibitions. It can also act as an emotional stabilizer. Wearing an opal is said to bring about loyalty and faithfulness.