Opal in Depth
Water, silica, and diffraction
Opal is not a crystal at all. It is hydrated amorphous silica, with anywhere from three to twenty-one percent water bound into it, and it has no crystal lattice. Its structure is a packing of microscopic silica spheres. When those spheres happen to be uniform in size and stacked in an orderly array, they diffract light and produce play-of-color, the flashing patches that make precious opal precious.
Sphere size determines which colors appear. Small spheres give blue and violet; larger ones are needed for red, which is why red-dominant opal costs the most. Material with no play-of-color is called common opal or potch. Australia produced the classic white and black opal from Coober Pedy and Lightning Ridge, and Ethiopia's Welo deposits have supplied enormous quantities of bright material since 2008.
The most fragile stone people wear
Opal is 5 to 6.5 on Mohs and it contains water, which is an unusual combination of problems. Dry heat, low humidity, and sudden temperature changes can cause crazing, a network of fine internal cracks that appears without warning and cannot be repaired. Never leave opal in a hot car, on a radiator, under a lamp, or in a safe deposit box for years without humidity.
Ethiopian Welo opal is hydrophane, meaning it actively absorbs water. Submerge it and it can go transparent and lose its play-of-color entirely, usually recovering over days as it dries, but sometimes cracking in the process. Never soak hydrophane opal, and be careful with hand lotion and dishwater. Australian opal is not hydrophane but still should not be soaked.
Doublets, triplets, and what you own
Much affordable opal jewelry is not solid. A doublet is a thin opal slice glued to a dark backing to deepen the color; a triplet adds a clear quartz or glass cap on top. Both look excellent and both fail if immersed, because water gets into the glue line and creates a cloudy fog that is permanent.
Ask whether a piece is solid before buying, and never put a doublet or triplet in water at all. Clean any opal with a barely damp soft cloth only. No ultrasonic, no steam, no salt, no chemicals.
Reputation and pairings
Opal is the October birthstone and is linked to Libra, Cancer, and Scorpio. The old superstition that opal brings bad luck is generally traced to a nineteenth-century Walter Scott novel and to jewelers whose stones kept cracking, which had more to do with its fragility than with fate.
Its traditional association is amplification of whatever is already present in you, which is why practitioners tend to recommend it for people who already know what they are working on. It pairs with moonstone for lunar work, with clear quartz sparingly, and with black tourmaline or hematite when the amplification needs a floor. Keep it stored apart from everything harder, in a soft pouch, away from heat.