When I was seriously considering a Kretchmer setting, I was very impressed with Charles Koll Jewelers here in San Diego. I can't seem to find a web site for them, but their phone is
(619) 298-5655. That's was all I could find.
Anyway, they said I could bring in my own stone, but it had to be a certain clairty grade, due to the pressure of the tension setting, Kretchmer didn't want to break the stone. They also said, Kretchmer does a test on each stone he sets first, and they warned that it COULD get sent back to us if it didn't pass.
However, I really, really wanted an Asscher cut stone, and they called Kretchmer (??) and said that they could not set that for me
I would call some other Kretchmer vendors. Sorry these guys are not closer.