Tumbled Opal

Common opal is hydrated silica - SiO₂ with several percent water held in its body - a silica that set as a gel and never crystallised into quartz. Amorphous, Mohs 5.5 to 6.5, softer than quartz.

It is the opal without play-of-colour: where precious opal flashes spectral fire, common, or "milky," opal glows softly and evenly, translucent white to faintly blue or honey. The glow is light scattering in the silica, not diffraction.

Because it holds water, opal can dry out: kept too hot or too dry over time, the surface may craze into fine cracks. It asks gentler care than the quartzes.

Asked often

Why doesn’t it flash colour like the opal I’ve seen?
Because this is common opal, the kind without play-of-colour. The flashing "fire" belongs to precious opal; common, or milky, opal glows softly and evenly instead.

Is it really part water?
Yes - opal is hydrated silica, holding several percent water in its body. That is why it glows softly, and why it asks to be kept from heat and long dryness.

How hard is it?
Mohs 5.5 to 6.5, softer than quartz and a little porous. Spare it knocks and harsh chemicals, and wipe it only with a soft, barely damp cloth.

How do I look after them?
Wipe with a soft, barely damp cloth, and keep them from heat, harsh chemicals, and long dryness, which can craze the surface. Spare them knocks; opal is softer than quartz.

1. Carried in a pocket or a bag, or held in the hand.
2. Kept in a small dish, on a desk, or on a shelf - each stone is a little different.
3. Wipe with a soft, barely damp cloth; keep them from heat, harsh chemicals, and long dryness.

Each stone is tumble-polished and unique, so no two are quite alike. Many keep a stone where they will see it each day, or carry one in a pocket; that is a matter of preference.

Pocket stones to keep and carry, gently. Opal holds water in its body - keep it from heat, harsh chemicals, and long dryness, wipe it with a soft, barely damp cloth, and spare it knocks.

• Stone: Common Opal · hydrated silica
• Hardness: Mohs 5.5–6.5
• Character: Milky white · soft glow (no play-of-colour)
• Form: Tumbled stones · polished, each unique
• Care: Wipe with a soft, barely damp cloth · keep from heat and chemicals

Opal is one of the few gem materials that never became a crystal: silica-rich water seeped into cavities and set as a solid gel, water and all. Most of the world’s opal comes from Australia, where it filled seams in ancient sedimentary rock; common opal is found far more widely than the precious kind.

The name descends through Latin opalus from older words for a precious stone, and Rome prized opal among the most valuable of gems. The soft glow of the common stone is light in silica - named plainly, without the fire of its rarer cousin.