Opal is the one popular gem that can change after you buy it. It contains water, it is soft, and some material develops a network of fine cracks called crazing. None of that makes opal a bad purchase — it makes the type of opal, and the seller’s disclosure, more important than with almost any other stone.
What opal is
Opal is hydrated silica: microscopic silica spheres with water between them. The water content varies widely — often somewhere between roughly 3 and 10 percent, and higher in some material. Where the spheres are uniform and stacked regularly, they diffract light and produce the flashes of colour known as play-of-colour. Where they are not, you get common opal, which has body colour but no play.
Opal sits at about 5.5 to 6.5 on the Mohs scale — softer than quartz, which means ordinary dust can scratch it — and its toughness is poor. It is a stone for pendants, earrings and occasional-wear rings rather than a daily knockabout piece.
The types, and what each one commits you to
| Type | What it is | Buying implication |
|---|---|---|
| Solid opal | One piece of opal | Most valuable; can be worked and repolished |
| Doublet | A thin opal layer glued to a dark backing | Cheaper; water can penetrate the glue line and cloud it |
| Triplet | Backing, thin opal, and a clear cap on top | Cheapest; the cap magnifies the colour; never immerse |
| Boulder opal | Opal still attached to its host ironstone | Natural, durable, and the backing is part of the stone |
| Hydrophane opal | Porous material, much of it from Ethiopia | Absorbs water and can go temporarily transparent; higher crazing risk |
Doublets and triplets are legitimate products, and they are also the most common thing sold without clear labelling. Look at the stone from the side: an assembled stone shows a straight join line. Ask the question directly and get the answer written on the invoice.
Crazing, and how to reduce the risk
Crazing is a network of fine internal or surface cracks that can develop over time, often associated with drying, heat or rapid changes in humidity. It cannot be repaired, and it is not always predictable. What you can do:
- Buy from a seller who states the origin and type, and who has held the stone long enough to know it is stable.
- Keep opal away from heat sources, direct sunlight for long periods, and dry environments such as a safe with a desiccant.
- Avoid sudden temperature changes — do not leave it in a hot car and then run it under cold water.
- With hydrophane material, avoid prolonged soaking; it can absorb liquids, including anything dissolved in them.
What drives value
- Play-of-colour — its brightness, the range of colours, and whether the pattern is visible from all directions. Red and orange play in a dark body is the most prized combination.
- Body tone — black opal (dark body) shows colour most dramatically and is the most valuable; white or light opal is more common.
- Pattern — broad, organised patterns generally rate above fine, pinfire specks.
- Solid versus assembled — a doublet is a fraction of the price of an equivalent solid stone.
- Clarity and shape — no visible cracks, no sandy or matrix inclusions where they interrupt the colour.
Care and setting
Never put opal in an ultrasonic or steam cleaner. Clean it with a soft damp cloth and mild soap, and dry it gently. Never immerse a doublet or triplet — water travels along the glue line and clouds the stone permanently. Store opal away from harder gems that will scratch it, and remove opal rings for anything physical.
For settings, a bezel protects the girdle and edges far better than prongs, and a low profile keeps the stone out of harm’s way. If a ring holding an opal ever needs sizing or repair, the stone must come out first — the heat of a torch is one of the few reliable ways to destroy one.
Frequently asked questions
Should I soak my opal in water or oil?
No. Oil and glycerine can be absorbed by porous material and change the appearance permanently, and soaking assembled stones ruins them. Ordinary humidity is enough for solid opal.
Is Ethiopian opal a bad buy?
Not at all — much of it is bright and well priced. It is more often hydrophane, so it can absorb water, temporarily lose its play-of-colour until it dries, and has a higher crazing risk than typical Australian material. Buy it knowing that.
Can crazing be fixed?
No. Polishing removes surface marks but not internal cracking. This is why storage conditions and buying from a knowledgeable seller matter so much.
Is opal too soft for an engagement ring?
It is on the soft, brittle end for a ring worn every day. It can work in a protective bezel on a hand that comes off for physical work — but expect to have it re-polished or replaced eventually, and consider a harder stone if that is unacceptable.
Sources
- GIA — opal description, quality factors and care — hydrated silica composition, Mohs range, doublets and triplets, cleaning limits.
- FTC Jewelry, Precious Metals, and Pewter Industries Guides, 16 CFR Part 23 — disclosure of assembled stones, treatments and special care requirements.