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Garnets, other heat sensitive stones and old Garnet

Precedence: bulk

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

You got it right. The glass is simply fused to the garnet, and then
the combination is cut after the materials are fused. Garnet is
formed in metamorphic geology, which means a lot of heat and
pressure, slowly applied. So heat by itself doesn’t hurt the garnet.
Normally, it’s heat shock, from overly rapid heating and cooling,
which causes unequal thermal expansion/contraction between the stone
surface and it’s core, to cause the cracking. In the doublets, the
thin sections are much less prone to such shock, and when the doublets
are made, it’s usually done in a furnace, which is a gentle
heatiung/cooling cycle. Note that different garnets may react
differently, and with any natural stone, the presence of inclusions,
which can have dramatically different rates of expansion, can make
otherwise heat tolerate stones self destruct. In the case of stones
formed in, for example, pegmatites, these are often formed from
hydrothermal processes, and inclusions can be water filled cavities
(such as emeralds, quartz gems, and any number of others). This can
happen too, in metamorphic geology, but is uncommon with the
almandite garnets used for the doublets, so these generally are heat
tolerant, especially ins smaller sizes.