The glycerine is not good for storing opals that will be cut. It is
hydrophilic (pulls water out of the opal), and makes the opal
structure less stable, and prone to cracking or crazing.
The chalky white could be potch which would normally be trimmed or
ground off, or it could be hydrophane opal as someone mentioned,
which only shows color and translucency when it’s wet. Or it could be
something that precipitated on the opal as it dried out. It will be
obvious once you start cutting which is the case.
Since the jars are 30 years old, the floating brownish gunk could be
rusty flakes from the lids of the jars deteriorating over time and
not be related to the opals at all.
Unfortunately, the glycerine may have damaged the opals. But you
have nothing to lose in using them for practice. If they survived the
glycerine, and survive cutting without cracking up, then you’ll
still need to let them sit for several months to a couple of years to
see what happens.
Overall, if it’s not mistreated, some Coober Pedy is stable and cuts
well; some is not stable and will craze. The depth at which it’s
mined may be a major factor, but there’s no way of telling now with
yours, except by cutting and watching.
You can go to the American Opal Society website, www.opalsociety.org
for a low-tech way to polish opals using wet/dry sandpaper on dowels
or wood blocks. Tim and Barbara Thomas have some common sense,
straight talk opal FAQs, and a guide to cutting opal on their
website, http://www.opal-tibara.com/ They also have photographs of
several types of Coober Pedy opal so you can see the wide variety
that is available.
Now, with all this bad news about your opal rough probably being
unstable, you can still clean it up, and if there’s color, put the
little bits that fall apart into little sealed vials filled
with…glycerin or mineral oil. They will reside there quite happily
as long as their container is sealed as a display item, or the tiny
vials, can be used as pendants.
Carol J. Bova