There are several factors here. one is that the sonic is a
highly effective cleaner. Sometimes you don’t really want that
much cleaning, such as with stones that have oil fillings or the
like. Another is that the sonic cleaner is putting considerable
energy into the solution. Some highly stressed or brittle
materials might shatter or crack, simply from that sonic energy,
kinda like that old memorex ad where Ella Fitzgerald hits the
right high note and a glass shatters… This is not common.
But it can happen to a few fragile stones. And finally, remember
that ultrasonics clean by causing cavitations, which is the
formation, expansion, and collapes of tiny bubbles. Very
rapidly, and concentrated at surfaces. The result is a rather
vigorous scrubbing action. Soft materials can literally have the
polish taken right off them. The degree to which this happens
depends on your cleaner. Many of the littler units simply don’t
have the power to do this very fast. They also, of course, take
a long time to remove even a few fingerprints… But the bigger
or more powerful units can damage some things quickly. Some
silver, especailly castings, is soft enough that the sonic will
leave frosted areas or whiteish swirl marks where it’s literally
scrubbed the polish right off. For a dramatic demonstration of
what the sonic can do, as well as a test of how effective a
sonic you have, put a piece of aluminum foil in there for a few
minutes. With a good ultrasonic, it will come out looking like a
sieve, with holes punched right through it.
for the most part, gold, platinum, and rolled/drawn/worked
silver items are OK. Most of the harder stones are also safe.
But to start your list of cautions, almost any stone you’d not
put in the pickle, probably shouldn’t spend a lot of time in the
ultrasonic. An exception to that is pearls. While pickle would
dissolve them rapidly and destructively, with the proper
cleaning solution, they won’t generally be damaged by an
ultrasonic (they’re pretty tough, actually) unless they are of
very low quality with very thin nacre. One exception to that is
mabe pearls. Some of them have very thin layers, and sometimes
an ultrasonic can destroy them. Not common, but I’ve had it
Some of this is relative. Most stones can take a brief "swish"
in the ultrasonic to remove a little rouge from the ring you just
touched up. Soaking them in it for 15 minutes might be another
One of the big red flags, though, is tanzanite. Because of
stresses sometimes built up in heat treating, tanzanites have a
(deserved) reputation for cracking or even shattering in an
ultrasonic. That’s not to say they all will, or even many of
them will. But enough of them might, that it’s generally felt
they should never go in an ultrasonic, even briefly, unless
you’re quite able and prepared to replace the thing. Silly risk.
Use a boil out solution instead.
Another obvious No-no is amber, especially if the solution has
ammonia. Destroys it quickly.
Stones that are oiled or otherwise treated, as with most
emeralds, risk having the oil come out. I use an ultrasonic
regularely with many emeralds, but only after a good examination
of the stone to evaluate that risk. Highly included or
particularely valuable ones don’t go in.
Same sort of thing with opals. the little run of the mill 6x4
ovals that cost a buck or so I don’t worry about. But the big
expensive ones, I don’t put in the sonic.
While turqoise or malachite and coral probably won’t be harmed
by brief cleaning, the solutions themselves might be harmful.
Use caution with these, and other such soft or porous stones.
Malachite and coral, if left in a powerful ultrasonic for too
long, can get dulled. Quickly fixed with a brief buffing, of
course, but avoidable.
Peridot can stand a brief cleaning, but don’t leave it in there
very long. It’s a bit soft to withstand that scrubbing,
especially if you’ve got a powerful cleaner.
And watch out for assembled or coated stones, like much of what’s
in costume jewelry. Ultrasonics are pretty good at getting
things unglued, or quickly seperating a rhinestone from it’s
foiled backing, etc.
Hope this helps. It’s not a complete or extensive list, but it
will give you a start on what to look for…