Opal is unlike every other gem on this site. It is not a crystal at all but a mineraloid, an amorphous form of hydrated silica containing anywhere from about 3% to 10% water (sometimes more). That water content and non-crystalline structure explain both its magic and its fragility.
Play-of-color: the reason opal exists
Precious opal is famous for play-of-color, the shifting flashes of spectral color that dance across the stone as you move it. This is not a dye or a coating. It is caused by microscopic silica spheres stacked in an orderly grid; when the spheres are uniform in size and arrangement, they diffract light into rainbow flashes. When they are irregular, the stone shows little or no play-of-color and is called common opal or "potch."
- White opal has a light body color.
- Black opal (famously from Lightning Ridge, Australia) has a dark body that makes the color flashes especially vivid, and is the most valuable type.
- Fire opal refers to a translucent orange-to-red *body color* (often from Mexico) and does not necessarily show play-of-color.
A delicate gem
At Mohs 5.5 to 6.5, opal is soft and fragile by gem standards, well below the ~7 threshold where everyday dust stops scratching a stone. More importantly, its water content makes it sensitive to heat, dryness, and sudden changes in temperature or humidity, any of which can cause fine internal cracking called crazing. Opal is happiest in pendants and earrings, or in protected ring settings worn occasionally, rather than in an everyday ring.
Ethiopian opal behaves differently
Much modern opal comes from Welo, Ethiopia. A lot of it is hydrophane, meaning it absorbs water. A hydrophane opal can temporarily change color or lose its play-of-color when wet, then recover as it dries. This is normal, documented behavior, but it means Ethiopian opal should be kept away from prolonged soaking, oils, and cleaning solutions.
Treatments and assembled stones
Because opal is porous and prized for dark bodies, several treatments and constructions are common and must be disclosed:
- Smoke and sugar-acid treatments darken pale Ethiopian opal to imitate black opal.
- Dyeing and impregnation with resin or oil.
- Doublets (a thin opal slice on a dark backing) and triplets (opal between a backing and a clear cap) are assembled stones, far cheaper than solid opal, and must be labeled as such.
Caring for opal
Clean solid opal only with a soft, slightly damp cloth. Avoid ultrasonic and steam cleaners, harsh chemicals, prolonged dry heat, and long soaking. Never store opal in a hot, dry place; some collectors keep them away from desiccants. Doublets and triplets must never be soaked, as water can seep between the layers.
No gem rewards understanding more than opal. Handle its softness and water content with respect, know whether yours is solid or assembled, and it will keep flashing its private galaxy of color for a lifetime.
