Hello,
Malachite is very soft and should never be cleaned with an ultra
sonic or any of the sort you should use a very mild soap. Anyway
you can return the polish on the stone with a buffing machine
with a buff charged with your favorite polish compound I like Zam
does a great job.
Oxidation on silver is normally for lower points on the piece
high points will wear off, You can use liver of sulfur to restore
it (Radio Shack stocks this cheaply buy the quart). or try
Griffith Silver-Black Jewelry supply house. CAUTION do not go
near Malachite with either of the chemicals. will damage the
stone.
Kenneth Sareyka
Kens Gems and Jewelry