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Garnets, other heat sensitive stones and old Garnet Doublets

Dear Friends, some weeks past were posts on working jewelry with
gemstones normally considered normally damaged by heat. Most postin=
seemed to opt for protection of such stones as garnet, etc., stones
other than the usual heat rule of diamond, ruby and sapphire.
Recently I have read articles on casting in place and indications
are that heat in itself is not so damaging as the uniform application
and controlled heating and cooling processes. Certainly, some stone=
will be damaged regardless.

A recent experiment at my bench was enlightening: Garnet doublets
likely from years before most of us were born are in supply where I
work. The store started in 1930 and inherited the stocks of an
earlier jewelry business. We have doublets in all gem colors. If you
are unaware of these simulated gems, the garnet doublet uses a slice
of garnet thick enough to cut the table of a stone. On the pavilion
is glass of suitable color for the simulant. The garnet color is
washed out in the thin slice and the pavilion color shows. Such a
doublet has a durable top surface and works pretty well as an early
form of simulated gemstone.

The experiment was simple. I placed the doublets garnet down on a
ceramic heating pad and put the torch to the glass pavilions. The
glass melted and formed a ball atop the garnet. What of the garnet?
Undamaged. No cracks, not a sign of heat damage. In a ring with a
retipping job a natural garnet would be fried and ruined. It seems
there is validity to the idea of uniform heat application and
gemstone damage.

Now, can anyone tell me how the garnet/glass doublets are made? I am
guessing a fusing of the glass to the garnet but do not know for
certain. The heat experiment tells me no form of cement is used to
join the glass and garnet tables.

Thanks for any input.=
Thomas. @Sp.T