Cutting, polishing, repairing opal: what a workshop can do
Opal knowledge · Workshop
Opal dull, scratched or broken: what can the workshop save?
A goldsmith can still cut or polish an opal years after you bought it. Whether it is worth doing depends not on the tools but on the damage: a dull stone is one case, a cracked stone another, and a broken stone another still.
Before you take the piece to a goldsmith, it pays to look at what actually happens at the cutting bench. After that you will know which repair is realistic, what the stone will lose in the process, and which hopes you should bury straight away.
How is an opal cut and polished?
Opal is cut as a cabochon, in other words domed rather than faceted: on diamond wheels under running water, from a coarse 260 grit in several stages up to 3,000, and then polished on leather or felt with cerium oxide or tin oxide. The water is not a side issue. Opal contains bound water and has no crystal lattice; if it heats up while being cut, it can dry out and develop fine cracks. At the cutting bench the simplest rule of the craft therefore applies: the stone stays cool and wet, always.
Why the cabochon and no facets? Because the play of colour runs across the surface and does not come from the depths the way it does in a diamond. Facets would only slice that surface up. The whole path from rough stone to setting is in How opal jewellery is made. Here it is about what comes after that, when a finished stone has to go back on the wheel.
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Assessment under the loupe
Look first, cut later: are they scratches, a crack or a break? Solid opal, doublet or triplet? That decides how much material may come off at all.
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Coarse grind with 260 grit
Only if the shape or a deep scratch requires it. The coarse diamond wheel takes off a noticeable amount of material, so use plenty of water and short passes.
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Fine grind up to 3,000 grit
Finer wheels stage by stage, each one taking away the marks of the last. After the final stage the stone has a silky sheen, but not yet a mirror.
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Polish with cerium oxide or tin oxide
On leather or felt, mixed with water, until the surface mirrors. This is where the gloss is born, not on the cutting wheel.
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Check and reset
Under the loupe: no crack opened, play of colour across the whole face, dome even. Then back into the setting, which often needs a slight press to close.
Can a dull opal be polished again?
Yes, usually: a dull or finely scratched opal is repolished, and only tenths of a millimetre are lost. Dull almost always means the surface is scored by countless tiny scratches, each one invisible, but together a grey veil that scatters light instead of mirroring it. The polish takes off exactly that top layer and lays the smooth face underneath bare again. The play of colour was never gone; it was only sitting under frosted glass.
Two causes you should rule out before the stone goes on the wheel. First a film of cream, soap and skin oil that comes off with lukewarm water; how to check that is under Opal care. Second water in the stone: an Ethiopian Welo opal turns milky when it has taken up moisture, and clears again as it dries on its own. Neither needs a workshop.
Where repolishing reaches its limit
On solid opal the limit is wide, because the whole stone is opal. On a doublet it is tight: the opal layer on the dark backing is often no thicker than a millimetre, and every pass over the wheel takes some of it away. Here you polish carefully and only as much as needed. On a triplet the case is different: you polish the quartz cap, not the opal. The cap takes it well, and the opal layer underneath stays untouched.
Deep scratches you can feel with a fingernail are no longer a job for polish. Then the whole surface has to be taken down to the bottom of the scratch, a true recut with a noticeable loss of weight. The stone becomes flatter, sometimes smaller too, and the setting has to be adjusted afterwards. At that point you weigh honestly whether the scratch may simply stay.
Can cracks in opal be repaired?
No, a drying crack cannot be healed: it can be stabilised if it sits deep, or ground away if it is shallow, and that is all. A crack is a split in the stone, and no process joins opal back together. Why opals crack when they dry too fast or take heat is set out in full under Is opal fragile.
If the crack sits only in the top layer, it can come off with a recut, with the same loss of material as a deep scratch. If it runs through the stone, what remains is stabilising: a thin resin is drawn into the crack and cured. It holds the stone together and makes the crack less conspicuous; the process promises no more than that. The stone then counts as treated, and that has to be said at any later sale. A crack that has not changed in years may, incidentally, simply stay; heat and dry air are then its only enemies.
| Damage | What the workshop can do | What is not possible |
|---|---|---|
| Dull gloss, fine scratches | Repolish, loss only tenths of a millimetre | On a doublet only to a limited extent; on a triplet only the quartz cap |
| Deep scratch | Recut down to the bottom of the scratch | Without loss of weight; the stone becomes flatter |
| Shallow drying crack | Grind away in a recut | On a thin opal layer, for instance a doublet |
| Deep drying crack | Stabilise with resin; the crack becomes less conspicuous | Heal; the stone then counts as treated |
| Broken | Recut the fragments separately | Glue as a repair; glued, it is a doublet |
| Ring too tight or too loose | Unset, alter, reset | Solder with the stone in the setting |
| Claw bent | Bend back; the stone stays in | Solder on a broken claw without unsetting |
Opal broken: what can you still do?
A broken opal is not lost, but it will never again be that one stone: the fragments are recut separately and become smaller stones. The question that always follows: can you not simply glue it? You can, only that is not a repair. The glue seam stays visible as a line where the play of colour breaks off, and from that moment the stone is an assembled stone. In the trade that is called a doublet, and that is how it must be named if it is ever sold.
The better path starts with a look at the pieces. A clean break through an oval ring stone often yields two pieces from which a round cabochon and a small pear are cut; a chip from the edge sometimes still becomes a stone for a slim ring. The setting of the old piece almost never fits afterwards, because every new stone is smaller. In return they are genuine stones with no seam, and play of colour to the edge.
What you can do at once: keep all the pieces, including chips, and try nothing with superglue. Glue residue otherwise has to come off first before any cutting can even start.
Why is the opal unset before soldering?
Because soldering heat destroys opal: silver and gold solder only flow at several hundred degrees, and at those temperatures the stone loses its water, clouds and cracks. That is why, for any work with the flame, the opal comes out of the setting, and it comes out first. This covers more repairs than people think: sizing a ring up or down, renewing a bail on a pendant, soldering on a broken claw, repairing a chain clasp when the stone sits right next to it.
Soldering heat: the fastest way to lose an opal
An opal ring that goes into the flame with the stone in place comes out cloudy and cracked, and that cannot be undone. When you hand a piece in for alteration, say clearly that an opal sits in it, and ask whether it will be unset first.
The same goes for the ultrasonic bath and the steam cleaner that belong to the final clean in many workshops: both stay off with opal. Vibration and heat can trigger cracks and, on doublets, loosen the glue.
Sizing an opal ring up therefore always means: open the bezel or bend the claws, stone out, saw the shank, solder in a piece of gold or silver, grind, polish, stone back, close the setting. That is more work than on a ring without a stone, and it is why the size matters so much before you buy. How to determine it reliably is under Measuring ring size.
Which setting protects the opal best?
The bezel: a closed metal rim catches knocks at the edge, and the edge is the place where a cabochon is most likely to chip. The bezel is also kind in repair. It can be opened and laid back after a recut of a stone that has become a little smaller. Claws show more of the stone and let more light in, but they hold only at four or six points. A bent claw is bent back without the stone coming out; a broken one is soldered on new, and for that it must.
If you are looking for a ring for every day, look in the Opal rings collection for a closed rim around the stone. For earrings and pendants, which knock less often, claws are unproblematic.
Three cases from the workshop: what worked and what did not?
Three pieces of the kind that land on the bench again and again. They show better than any rule what you may expect of a repair.
Case 1: the dull heirloom
A ring from an estate, white opal in gold, worn for decades and dull as frosted glass. Under the loupe: countless fine scratches, not a single crack. The stone was unset, repolished on felt and reset. Height lost: not measurable. The play of colour was back at once; only the bezel needed a little extra press.
Case 2: the ring that was too tight
An engagement ring with opal in a bezel, two sizes too small. Bezel opened, stone out, shank sawn, a piece of gold soldered in, ground, polished, stone back, bezel pressed home. Without unsetting, the opal would have come out of the flame cloudy and cracked. As it is, you cannot tell the ring has been altered, except that it now fits.
Case 3: the broken stone
A pendant falls on tiles, the oval opal lies in two pieces. The question about gluing came at once, the answer was honest: glued, it would be a doublet with a visible seam. Instead a small pear was cut from the larger piece, and a round stone for a slim ring from the smaller. Two stones instead of one, smaller, but genuine and with no seam.
What all three cases share is the first step: the assessment under the loupe, before anyone picks up a tool. What happens after that is discussed beforehand, with what the stone will gain and what it will lose.
Jewellery from our workshop: every piece comes with a GSB certificate and 5 years of guarantee. If something is not right on a piece of ours, talk to us before you have it worked on elsewhere. We know the stone and the setting.
Common questions on cutting, polishing and repair
Can you polish an opal?
Yes. A dull or finely scratched opal is repolished on leather or felt with cerium oxide or tin oxide, wet and cool.
My opal has gone dull. What should I do?
First check whether it is only a film of cream and soap, or whether a Welo opal has taken up water; both sort themselves out without a workshop. If the stone stays dull after washing and drying, they are scratches, and then repolishing at a goldsmith's will help.
Can cracks in opal be repaired?
No. A drying crack cannot be healed.
Opal broken: what can I do?
Keep all the pieces and take them to a goldsmith. The fragments are recut separately and become smaller stones.
Can an opal ring be sized up?
Yes, but only with unsetting. The stone comes out of the setting, the shank is sawn, a piece of metal is soldered in, then it is polished and the opal is reset.
May opal jewellery go in an ultrasonic bath?
On no account, and not under the steam cleaner either. Vibration and heat can trigger cracks and, on doublets, loosen the glue.
If you are unsure or have questions, you are welcome to contact us at any time, even if you did not buy the opal from us. Contact form, email cs@libertyangels.de or call +49 171 1603048.
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Written by Anton Kehler
Gründer FAERELLI & Liberty Angels
