I was told GIA did not recommend ultrasonic because of possible
flaws in stones, expecially diamonds. Can you clarify?
GIA classes stones as: safe, usually safe, risky & never, as far
as suitability for the ultrasonic cleaner. All classifications
are based on stones with no inclusions. The presence of
inclusions drops a stone to the risky or never class, depending
on the size & type of inclusion.
It's never really safe to make blanket statements, especially
about things in the natural world, but here's a gross overview.
There are a number of stones that should NEVER be placed in an
ultrasonic cleaner; oiled, highly included & those showing
fractures. There's a large list of stones that even if they are
(or appear) perfect should never be put in an ultrasonic, most
notably, emerald. Additionally, no assembled (doublets, triplets
etc) stone should be subjected to ultrasonic cleaning.
Diamonds, corundum, garnets & spinels are usually safe. Jade,
hematite, YAG & CZ are safe.
All the rest of the stones are classed as either risky or never.