The name derived from an Indian word for “stone”. It is divided into three groups: the opalescent precious opals, the yellow-red fire opals and the common opals. Their physical properties vary considerably.

Color: White, gray, blue, green, orange
Color of streak: White
Mohs’ hardness: 5.5 – 6.5
Specific gravity: 1.98 – 2.20
Crystal system: Amorphous ; kidney or grape shaped aggregates
Chemical composition: hydrous silicon dioxide
Transparency: Transparent to opaque
Refractive index: 1.44- 1.46
Double refraction: none

Precious Opal

The special characteristic of these gems is their opalescence, a rainbow-like iridescence which changes with the angle of observation. Until the 1960s, this was thought to be caused by the refraction of light from the thin surface layers. The real cause was discovered under the electron microscope using a magnification of 20,000 : tiny spheres (0.001 of a millimeter in diameter) of the mineral cristobalite layered in siliceous jelly cause  the reflection or interference appearances. Strictly speaking, precious opal is not really amorphous.

Opal always contains water; the content varies but it can be as much as 30%. It can happen that in the course of the time, the stone loses water, cracks and the opalescence diminishes. This can, at least temporarily, be restored by saturation with oil or water. The aging process is avoided and the opalescence increased when stored in moist absorbent cotton (cotton wool). Care must be taken during setting. A little heat can evaporate the water. Opal is also sensitive to pressure and knocks as well as being affected by acids and alkalies.

Fire Opal

Named after its orange color. It does not opalesce and is usually milky and turbid. The best qualities are clear and transparent. They are very sensitive to every stress. Important deposits are in Mexico (Hidalgo and Queretaro Provinces); also in Brazil, Guatemala, Honduras, US, Western Australia and Turkey (Simavopal). Glass imitations are found in the market.

Girasol: (“Sunflower”) nearly colorless, transparent variety with bluish sheen and red play of color. A name sometimes given to girasol or fire opal is sun opal.

Mexican Water Opal: variety from Mexico, colorless or slightly brown, transparent with unicolored schiller.