Have you ever looked closely at a a hand-chipped stone arrowhead? Those are
pressure flakes - originating at a point of excess pressure and flaking
conchoidially outward from there. With that in mind now look at the shape
of the flakes that came off the opal. Get the picture? What you have is an
engineering issue: excess pressure on the stone.This may have two causes.
One lies with the bezel. Bear in mind that opal is already a fragile sort
of stone. Thus you want to put the holding pressure on it gently, evenly
distributed all around, and in closing the bezel, "patting" the metal
against the stone like butter. This is easy with malleable metals like 22 K
or fine silver. But 14 K is resilient and springy and you have to force it.
This means the inside inside the bezel instead of being buttered up against
the stone all over, the metal may be in contact at only one or two or three
or some other number of individual points on the stone and with that you
have so many individual pressure concentrations that press against the opal
like pebbles under your bare foot.
The other cause may like with the shape of the cut opal. With opal, sharp
corners and flanks that are cut too thin are trouble waiting to happen -
again because they concentrate pressure into small areas.
Both these factors may work together. Hard metal plus thin stone? Give that
job to somebody else.
I might add that opal is about my favorite stone; I've cut a lot and set a
lot, so these observations come at first hand. There's no other stone quite
as mysterious and magical and I believe it is for that reason that opal has
acquired such a baggage of myth and mystification - in a sort of unintended
compliment to the mysterious nature of the stone. For that reason it seems
to also draw the "mystificateurs" who pronounce, usually ex cathedra, stuff
like, oh you should never keep it in water, oh you should always keep in
water, never let it dry out, always let it dry out for at least six months,
always put it in glycerine, never put it in glycerine, etc. etc. It's
hocus-pocus. Yours being an Australian opal water is about as likely to
affect it as are the gravity waves from Jupiter.