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Opal indeed has many myths around it. I've cut a great many stones
mostly Australian solids: crystals, whites, some blacks. I've made
doublets and triplets which can also be fun.
It seems like locale is the primary predictor of stability in opal.
The Aussie stuff seems generally stable, though I'm sure there are
exceptions (one of which I'll mention later). Boulder also seems
stable, albeit the layers of opal can be so thin that they crack
Mexican opal is surely unpredictable and may craze quite a long time
after cutting. I hate it when it does that. I had one pull that
little trick 6 or 8 months after cutting.
Idaho and Nevada I haven't cut, but have been told are frequently
unstable although I understand some Virgin Valley is cuttable. Same
is true for Honduran and Brazilian. Some of that stuff will go
completely white when out of water. In common opal without fire but
that is till quite pretty, Andean is mostly unstable, but the newish
pink Baja California find seems stable. I've had some on my bench
for months will no ill effects. I have also heard that the
relatively newer Brazilian find with fire is more stable, though I
have no proof of that.
So most of this is what I've heard or read. But the Aussie, old
reliable is still just that, relative to the others, reliable.
All that having been said, I find that opal is rather fragile. It
is not only fairly soft but also highly temperature sensitive,
especially heat and extreme cold, like below zero.
It also cracks fairly easily when struck which makes it
inappropriate in a ring for people who insist on using their hands a
great deal while wearing the ring. On the other hand (no pun
intended, I'm sure), I have a bezel set doublet in a ring I made for
myself years ago.
I wear it frequently and it's still whole. I know it gets some
abuse because I have to polish out scratches from time to time. I
did put a backing in the bezel with a cushion of rubber. That might
have helped it.
I have to say in general that I think opal is especially
inappropriate for prong set rings. I know many people like that,
but it seems too vulnerable for such a soft stone. It is after all
not that much harder than solid turquoise. Also there seems to be
too much potential for stress at too few places. One good rap can
do it in.
Still as long as the buyer is properly warned, I don't see how you
can prevent people from taking a risk.
One other thing with opal that can be a problem. Once older material
has been scratched, even the Aussie stuff, polishing it can make it
unstable. I don't know why that is but I've had more than one
person warn me off messing with older set opal.
All of that personal experience and hearsay now laid out, I can say
that I really do love opal. It was the first species of stone I
started cutting and the fascination has never completely worn off.
The orientation on opal and problems associated with cutting it
makes uncovering a true gem such a thrill.
And also exceptionally good material is so rare, especially in the
Lighting Ridge black.