I don’t know. I don’t think anyone can really answer this question as no one really knows whether crystals actually have a healing power! Please tell me if I’m wrong…
I don’t think it’s likely to damage the energy of the crystals, since the energy / vibrational level is something that occurs at atom level. As it is crystal hangers you are making – then when they are complete, hang them up where they can be exposed to sunlight / moonlight as appropriate and that should put your mind at rest. Crystals need to be cleaned every once in a while anyway – the “heating” may even help : )
Did crystals have healing powers to begin with? They can’t lose what they never had!
Crystals are prone to crack during exposure to rapid heat/cold change and quartz crystal is arguably the most fragile in this regard, being a poor conductor of heat or cold.
No need to worry. Since the only healing properties of crystals lays in the mine of the user, heat will not affect them.
All you are likely to do is break the latice structure.
Why would you put them in the oven in the first place?