I always make bezels too small, but deliberately so. Close, but on the
small side (last thing you want is a rattling good fit, right?). Then,
to bring it to size I just slip it over a short length of steel rod which
I rest across the open jaws of the vice, and gently tap it in a few places
with a light hammer. You can easily bring it up to size without causing
any serious surface marks. When it’s the right size clean it up all round
with wet and dry paper used wet in the normal way, before soldering to the
base.
Two caveats. I use plain strip for my bezels, not the commercial bezel
strip that gives a ledge inside for the stone. That stuff would be more
difficult to bring to size by hammering. Secondly, I’ve only used this on
round and oval stones. But I should imagine that the method would work
for stones with flat sides to (as per the original query), if you use a
piece of square section rod inside the bezel when you hammer it.
Gentle hammer taps, gentle taps.
Kevin
Kevin Eva, Northern England, UK
@Kevin_Eva (home)