First off, I can start to see a common theme for the sorts of
materials that ultrasonic cleaners are contra-indicated for - they
all have microscopic cracks - thanks Jo for that list.
Some have extensive microscopic voids or cracks - or lamella - like
malachite and pearls, some may have a few microcracks.
Some are surprising - emerald and sapphire for example. it may not
be as simple as microcracks though, the shape of the crack and the
brittleness of the crystal (surface free energy -
may play as large a role as anything...
wrt the opinion that opals are not crystalline, this is wrong - or,
For starters, there are various "degrees of order" between
crystalline and truly amorphous materials - opals are somewhere along
that spectrum, made more complicated because it is has a microscopic
structure as well as an atomic structure (most "crystals" have atomic
structure only) - the spheres that make up opal are crystalline - but
not "fullly" crystalline - they have a higher degree of disorder than
say quartz. The matrix that infills the space between the spheres is
Regardless, it's not the amorphousness that is the problem. Can you
clean glass beads? if so, being non-crystalline is not the problem
nor a common theme
nor is being semi-crystalline - c. f. chalcedony and agate - both
structurally and chemically similar to opals.
The treatment of opals with oil and their ability to gain and lose
water may play a role, especially if there is shape change involved.
Heat may play a role - once again especially if heat causes volume
changes - or loss of water and associated volume changes. especially
shrinkage which leads to easy crack propagation.
My gut feeling is that the combination of low toughness materials
(surface free energy) plus microcracks, aggravated by any treatments
or enhancements (that are reversed or interact with the cleaning
solvent or cavitation, causing surface shrinkage) are the key