David Geller is correct in presenting the little rule of thumb of
the industry, “red, white & blue” can take heat . . .
Please remember only the colors of the American flag can take heat: Red, white & blue (diamond ruby & sapphire. Faceted at that.
However, there are a few exceptions you need to be aware of, and not
paying attention in this regard can become very expensive.
Hmong Su (please forgive my spelling if it’s wrong) rubies are a
variety, quite common on the market, that are flux filled. They
have a lot of fractures, much like emerald, and they are heated in a
sort of glass, a flux, which penetrates these creatures and makes
them look more presentable. They have a nice color, but heating
them again is an iffy proposition. Look closely with a loupe. If
you see what look like cracks, filled but polished out a little, you
might be looking at Hmong Su. The filler material is softer than
the corundum, and in polishing, it wears down to a lower level.
David mentions faceted, meaning here that cab rubies don’t always
survive heat well, often due to being badly included.
Most sapphires today are often heated to diffuse color striations
throughout the stone for better appearance. But a natural sapphire
may not have been heated. It may also contain an "negative crystal"
or cavity that is filled with gases. Heating these can often cause
such bubbles to expand and crack the stone. A microscope may detect
these, if you know what to look for, but a good rule of thumb is “if
you can’t afford to replace it, don’t take chances, remove and
re-set”. Also, heating sapphires for even moderately prolonged
times when they are covered with boron containing fluxes (of which
most of the ones we use are) can etch the surface of the stone.
They can be re-polished, but it’s more time and expense. Try to
keep the boric acid and flux off the sapphire, and don’t get them
glowing red if you can help it. Also, natural star sapphires, black
or otherwise, shouldn’t be heated. They won’t like it. Linde type
man made sapphires can take anything short of a nuclear blast. They
change color when you heat them, then change back again (look for a
script “L” etched on the bottom).
There are filled diamonds, and supposedly the newer fillers can
withstand the heat of re-tipping, but the older ones won’t.
Sometimes you can see a “flash effect” in these stones, which is a
sort of rainbow looking cleavage in the stone that can be seen from
certain angles. In my repair business, I inform my accounts that I
won’t guarantee diamonds that are filled unless I’ve been informed
ahead of time. Badly included stones can also get worse from
heating. It is also possible to overheat a diamond. If you get
them glowing orange, you may notice when they cool down that it
appears you can’t get all the flux off the diamond. It’ll look
frosted, and it is. You can, of course, have it re-polished, but it
costs, and you lose diamond weight. If you’re heating the diamond
beyond a barely visible red, you need to look at your soldering
technique. Either there’s to much heat sink or you’re not on clean,
properly fluxed metal. You need to get good at detecting CZ’s from
diamonds. CZ’s are cheap, but your time used to replace them isn’t,
and you might have to do a lot of work to get the new one set and
looking like it did before. Get an old one and heat it with a torch
and quench it, so you’ll know what it looks like before you have a
heart attack from heating one unawares. And don’t quench diamonds.
You can usually get away with it, but it’s a bad habit, and one day
you’ll quench a sapphire or ruby by mistake and they don’t like that
at all. Moissonite is another story. When they’re set, they’re
difficult to detect, unless its an old one and that funny greenish
tint looks suspicious. When you heat them, they glow yellow-green.
I’ve not tried to see how much heat they can take, and they really
aren’t that cheap to replace either. If it glows, I stop. You can
get detectors to identify CZ and detectors for Moissonite, but I
don’t think there is one machine that detects both. Again, that
disclaimer is appropriate.
As a general rule of thumb, when it doubt, play it safe. A little
time spent can be a lot of time saved. I have encountered EVERY
SINGLE ONE! one of these problems first hand. I heard later what I
had done wrong. That’s a good way to remember!
David L. Huffman