what's the best way to oil opals? I have some rough that has
cracked, and would cheerfully disclose the treatment.
The best way to oil opals is to NOT oil opals. All it does is get
them oily. Oiling opals that have become somewhat worn and abraded
may temporarily make the surface scratches less visible, but it
doesn't fix that, just temporarily hides it. Actual cracks are not
helped at all by oiling, and may even cause discoloration if
impurities or whatever in the oil seeps into the cracks. Oil on opals
is not really a treatment per se, only a means to temporarily delay
any drying out of the stones that is behind the cracking and crazing
that sometimes occurs. And for this, you pretty much need to be
storing the opals immersed in the oil. Once removed a thin layer of
oil no longer protects anything.
The real problem is simply that some opals, especially uncut rough,
is prone to cracking as it sits in air, due to drying out, which
occurs faster at the surface than the interior, causing the surface to
slightly shrink while the interior has not, resulting in surface
cracks. You can't practically prevent this other than by storing the
opals covered, which is actually best done with just water, not oil.
And it's only a delaying tactic, not a cure or prevention.
Many people who cut opal simply expect to store it either as rough,
or cut stones, for a while so that those stones which may be prone to
cracking will do their thing and can be weeded out.
Again, the whole thing about oiling opals is that this is a popular,
widely believed by the public, old wives tale with no actual
gemological basis. Just don't do it. It's a waste of time, does no
good, and might even cause problems.
Peter Rowe G.G.