This is a bit of a mystery to me as well. If it's made of metal
clay, then jewelry cleaner should definitely have done the job.
Actually, when you first described the incident, I had thought that
the bead had actually been plated first and that the ammonia had
destroyed the outer layer.
But if it's truly metal clay (and that has yet to be confirmed)
simple jewelry polish (not cleaner, but polish) should restore the
shine. The important thing, now is to find out what it's really made
Art Clay World, USA, Inc.