Let me say one thing concerning the heating of Agates and Jaspers,
when heating stones, they will become brittle and harder as well as
change color and clarify in some material. Even for the low
temperature you anticipate, I would not apply heat to very thin
In regard to heating stones;
Normally if I am ‘cooking’ Agate, I do it at around 450 to 550 F for
6 hours, and 24 hours for cool down with the oven closed. I do not
think 10 to 20 minutes at 250 F would cause a problem. But as with
all things, don’t go throwing all your work in the oven at once. Even
when you change material, I would only do one piece and check the
results before doing a batch.
I have not heated Jaspers except rarely, but with the nature of the
stone, I don’t think it would be an issue at that temperature. Same
note applies as above.
Turquoise, I have no idea, I have never cooked it or heated it. But
I will say if it is treated material, I would not do it. Epoxy and a
lot of the stabilizers do not react well to elevated temperatures,
even one of so moderate a level.
Overall, I would not worry about the quartz based, I would not heat
the turquoise unless it was material you found yourself.