I *think* we're talking about enhanced or dyed crystals. Many citrines, for example, are heat-enhanced--it brings out a darker orangey shade. Amethysts too often get a bit of heat treatment to bring out some more color.
There are also the aura quartzes--like 'aqua aura' and 'angel aura' quartz, which have precious metals heat bonded to them.
And there are just the plain old dyed rocks. Lots of agates take dye well.
In my experience, the first two categories don't seem bothered by the 'enhancement' process. Citrine, especially, almost *likes* to be heated. Opals, if treated gently, can also be treated this way as well with no ill effects.
The only group I'm wary of are the dyed stones. I love agates, and find them the real earthy workhorses of the crystal world (quartz is nice, but can really amp up energy, while agates keep you grounded) and while the dye doesn't injure the stone, it doesnt' seem to help it. For example--I have a piece of agate that was dyed blue. Originally, it was clear and shades of brown. As regular old agate, it's fine, but if I try to work with it as a 'blue' stone or a blue agate, it doesn't wanna go.
SQ