ruby sapphire diamond only. I've seen people solder on some garnets but I won't.
There are caveats to heating those stones too. Diamonds that have
been “enhanced” by means of laser drilling and filling with glass
should not be heated. It won’t destroy the diamond, but the glass
filling can ooze out or at least discolor. I hesitate to heat any
diamond that is heavily included too. And there’s a certain kind of
glass filled ruby that you can’t heat either. p Corundum cabachons
shouldn’t be heated because they have much less integrity to the
crystal structure and will often split or pieces will flake off. Now
for saphires. Most saphires have been heated before cutting to even
out the color banding that is often present. But natural blues that
have never been heated can contain “negative crystals” filled with
water. These will often split if heated with a torch. How you
recognize these situations is another long story for another post.
So for corundum, it's not advisable to put ANY coating on prior to heating? Just clean the stone prior and heat gently, right?
Yest and no. A little boric acid on the stone won’t do much as long
as you don’t get the stone that hot, like glowing red. But in bright
light, it’s not always obvious how hot a stone is getting. The paste
type fluxes have a much more dramatic effect and should not be
present on the stones surface when you heat it, say, for re-tipping.
Even so, it depends on the stone. As for garnets, 2 or 3 millimeter
stones seem to survive pretty well, but bigger than that it’s about
50/50 odds they’ll fracture internally.
By the way, most synthetic stones, like the ones in mothers rings,
can take torch heat, except the green ones. They’re usually composite
stones (and don’t put the older green ones in the pickle or you’ll
see the glue seam around the girdle when you take them out).
Nowadays, the white stones in mo’s rings are usually CZ’s, and
they’ll take heat if you bring them up very slowly and let them cool
slowly, but it takes practice.
There are other stones that can be heated, but my rule of thumb is
this: If I either don’t want to spend the money to replace it, or
it’s going to be difficult to get a new stone back in the setting,
I’d refrain from heating it.
David L. Huffman