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About heat hardening silver

    Any concerns about color change in stones like amethyst, topaz,
etc.? Should heat hardening be done before setting stones? 

A brief trip to 550F should not effect an Amethyst.But to hold it for
an extended period at this temperature can fade the color of the
stone.Here are the significant temperatures and reactions on Amethyst
to heat treat.390C and dark Amethsyt lightens.450C to 550C and
Amethyst turns to Citrine.Over 550C Amethyst can go to clear
Quartz.The progression is Amethyst to clear,to Citrine,to clear again.
Imperial Topaz,peach/orange/red variety from Ouro Preto,MG,Brazil
would be unchanged on heating at 550F.But Blue Topaz,understanding
that all Blue Topaz is irradiated,even the very rare stone deemed to
have occured Blue in nature,were exposed to irradiation in nature,will
fade and eventually go to white at 550F.Heat reverts the effects of
irradiation. Mark Liccini