Opal
Opal is the one stone in this library that can change in front of you — and the one where the care advice is not optional politeness.
What causes opal's play-of-colour?
Opal is hydrated silica: microscopic spheres of silica packed in a three-dimensional grid, with water between them. When those spheres are uniform in size and neatly stacked, they diffract light like a natural grating, and you get play-of-colour — flashes of spectral colour that move as the stone moves. Where the spheres are irregular, you get common opal, which is opaque and has no play at all.
The sphere size determines which colours appear. Small spheres give blue and violet; larger ones are needed for red, which is why strong red play is the rarest and most expensive.
Can opal be damaged by water?
Most Ethiopian opal is hydrophane, meaning it is porous enough to absorb water. Submerge it and it will take water in, often going temporarily transparent and losing its play-of-colour entirely. It usually returns as the stone dries, which can take hours or days.
Usually. Rapid absorption and drying can also craze or crack a stone, and absorbed oils, soaps or dyes may not come out at all. So: no swimming, no washing up, no showering in it, and no soaking to clean it. This is not fussiness — it is the single most common way Ethiopian opal is ruined.
Australian opal is generally non-hydrophane and much less sensitive to water, though the rest of the care still applies.
Is opal durable enough for daily wear?
Mohs 5.5 to 6.5, with real sensitivity to knocks and to sudden temperature change. Opal contains water; heat it fast or dry it out over years in a hot, arid environment and it can craze — a network of fine surface cracks that cannot be polished out.
None of this makes opal unwearable. It makes it a stone for pendants and earrings more readily than for a ring worn while working with your hands.
How do you care for opal?
A soft, barely damp cloth and nothing else. No ultrasonic, no steam, no solvents, no jewellery dip. Put it on after perfume and hairspray. Store it away from direct heat and out of a dry safe for long periods.
Why is opal October's birthstone?
October's birthstone, and the subject of a persistent superstition about bad luck that traces largely to a nineteenth-century novel rather than to anything older. The stone is fine. Read the care notes and it will outlive you.