Here is something most jewellers will never tell you across the counter: opal is technically not a crystal. It is one of the few gems on Earth with no crystal structure at all. And the finest of it, the kind that throws fire in blue, green and red, comes almost entirely from one place. Australian opal accounts for the vast majority of the world's gem-quality supply, which means the stone that breaks the rules of mineralogy is also, more or less, ours.
I have spent close to twenty years with stones in my hands, and opal still surprises me. So let me explain what it actually is, why it behaves the way it does, and how to look after it honestly, without the usual sales gloss.
Why Australian opal is not technically a crystal
A crystal, in the strict sense, is a solid whose atoms are arranged in an orderly, repeating lattice. Diamond, sapphire, emerald, quartz: all crystals. Opal is not. Chemically it is hydrated silica, SiO2·nH2O, but those atoms never settle into a repeating structure. Opal is amorphous. Because of that, gemmologists don't even call it a true mineral. The correct term is a mineraloid.
So when someone asks is opal a crystal, the precise answer is no. It is silica that froze mid-decision. Under the surface it is built from countless microscopic spheres of silica, packed together in stacks, with tiny gaps between them. That hidden architecture is the whole reason opal does what it does.
The play-of-colour, explained properly
The shifting fire in an opal has a name. Gemmologists call it play-of-colour, and the old harlequin term for it is arlecchinamento. It is not a pigment and it is not a trick of polish. It is diffraction.
Those silica spheres sit in a regular grid, and when white light enters the stone it bends around them and splits into spectral colours, the same physics that makes a soap film or a butterfly wing shimmer. The size of the spheres decides which colours you see. Larger spheres, around 300 nanometres across, can throw red, orange, green and yellow. Smaller ones manage only blue. This is why a true red-on-black opal is so rare and so prized: the stone had to grow its spheres to an unusually large and even size.
A few honest specifics worth knowing:
- Water content ranges from 1 to 21 percent. Gem-quality opal usually holds around 6 to 10 percent. The stone literally contains water.
- Refractive index is low, roughly 1.45, lower than almost any other gem you'll meet.
- Hardness is only 5 to 6 on the Mohs scale. That is soft. Quartz, which is in everyday dust, will scratch it.
How opal forms, and why Australia
Opal is born from water, slowly. Silica-rich groundwater, what geologists call vadose or percolating water, seeps down through sandstone and clay, then gradually evaporates. The dissolved silica concentrates into a gel, settles into cavities and seams, and over an almost unreasonable stretch of time, it hardens. It has been estimated that an opal grows roughly one centimetre in five million years.
It forms in regular nodules that miners on the fields call nobbies, and in thin veins along the contact between clay and the sandstone above it, often around thirty metres down. Sometimes it replaces what was already there, filling the shape of an ancient shell, a fragment of wood, even bone. Those opalised fossils are extraordinary, and collectors and museums chase them.
Australia is the heartland for a reason that comes down to deep time. Across the late Cretaceous and early Tertiary, vast areas of the continent sat under an inland sea, then weathered intensely over tens of millions of years. That combination of silica-bearing sediment, patient water and geological quiet is rare. It is why the great fields, Lightning Ridge, Coober Pedy and the rest, sit where they do, and why black opal from Lightning Ridge, with its dark body tone that makes the colour leap, is among the most coveted gem material on the planet. Opal turns up elsewhere too, in Brazil, Mexico, Honduras, Indonesia and the United States, but for fine gem opal the centre of gravity is here.
Caring for opal, the honest version
This is where I differ from a lot of opal jewellery sellers, who would rather you didn't think too hard about it. Opal is soft and it holds water, and both facts matter for how you wear it.
Because it carries water, opal can dry out. If it loses moisture too fast, through heat or very dry air, it can craze, developing a fine network of internal cracks. So a few firm rules:
- Never put opal in an ultrasonic or steam cleaner. Clean it gently with a soft, damp cloth.
- Keep it away from heat, harsh chemicals, perfume and household cleaners. It is porous and will drink them in.
- Protect it from knocks. At a hardness of 5 to 6 it scratches and chips far more easily than a sapphire or diamond.
- As a daily ring stone it needs a protective setting, a bezel or a guarded mount. A solitaire opal on a hand that does real work is asking a lot of a soft stone. As a pendant or earrings, where it lives a quieter life, it is far happier.
None of this is a reason to avoid opal. It is a reason to understand it. A well-set, well-loved opal will outlast all of us. It simply asks to be treated as what it is: a piece of ancient Australian water, caught in silica, doing something no crystal can.
If you'd like to see opal chosen and set by someone who reads the stone before they cut it, that is exactly the work I do.
