Opals are milky white or colorless gemstones that reflect other colors. They are composed of silica, or silicon dioxide, a compound of silicon which is also found in sand, quartz and flint.

Although opals are basically colorless, they are rarely found to be so, as various impurities which spread throughout the gem give it dull tinges of color, from yellow or red, when iron oxide is present, to black, where there are manganese oxides and organic carbon. If an opal is milky white or gray, there will be many tiny gas-filled cavities inside it.

The black opal, which is an intense gray or blue or near-black, is seldom found and, for this reason, particularly valuable. The lighter white opal or the yellow, orange or red fire opal are far more frequently mined. Transparent opals reflect changing colors as the stone is turned. This is caused by the play of light on tiny cracks and other imperfections inside the stone. Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.

Australia has the finest gem opals, most of them mined in Queensland and New South Wales. In particular, the Lightning Ridge field is renowned for its black stones. High quality opals are also mined in Mexico, India and Slovakia.

Highly prized by the Romans, who considered it to be inferior only to the emerald, and regarded as lucky in the Middle Ages, the opal is nowadays sometimes considered to bring bad luck. Its presence in early Northern European mythology, where it was used by sorcerers to weave black magic and cast evil spells, may be associated with this conviction.