Treatments, enhancements and outright fakes
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A market with high prices, no certification and material that is hard to test invites exactly what you would expect.
Three categories
Treatment, which changes a genuine specimen: heating, irradiation, dyeing, coating.
Enhancement, which adds something: a glued-on crystal, a manufactured coating, an artificial surface.
Fake, which is not what it is sold as at all: manufactured material, a different species, a composite presented as natural.
The first is often legitimate and always disclosable. The third is fraud.
Treatments
Heating, which changes color in several species and is long-established for some gem material.
Irradiation, which produces colors that are frequently unstable in light, so an irradiated specimen is both treated and fugitive.
Dyeing, common in porous material: agate, howlite, magnesite.
Coating, which produces the metallic iridescent surfaces on quartz sold under invented names. Those are a manufactured coating on a natural crystal.
The FTC guides for the jewelry trade require treatment disclosure for gem material, and the same expectation applies by custom on the specimen side even where those guides do not reach.
Enhancements and fabrications
Crystals glued onto matrix they did not grow on, which is the commonest fabrication and can be convincing.
Manufactured crystals sold as natural. Several species are grown commercially and some reach the specimen market.
Composites assembled from several specimens.
Relabeled localities, which is a fake of the paper rather than the stone and is the hardest to detect.
How to protect yourself
Buy from dealers with a reputation to lose, which is most of the answer.
Look at the contact points: a glued crystal usually sits wrong, and the junction shows under magnification and under UV.
Be suspicious of perfect, of cheap, and of a species suddenly abundant.
Ask directly whether anything has been done to it. A dealer who answers precisely is telling you something; one who is vague is telling you something else.
And when you sell, disclose everything you know. You are the dealer in that transaction.