Two things mislead in every mineral listing — size and colour — and both are fixable with a question.
The questions
"Can you put it on a scale and hold something ordinary beside it?" Specimens photographed alone look twice their size. This is the most common disappointment in the category and the easiest to prevent. Non-answer: "It's a good size."
"What species is it, and how sure are you?" "Sold to me as X, I cannot confirm it" is an honest answer and should be priced as one.
"Where is it from?" Locality often matters more to collectors than size.
"Has it been treated?" Dyed, heated, irradiated, stabilised, coated, reconstituted. All normal, all declarable. Undeclared is the dispute.
"Can I see it in daylight, turned?" Colour under a show light is not colour.
"Any chips, repairs or contact points?" A contact point is how the specimen grew; a chip is damage. Ask which.
One thing to check
Coral, ivory and the shells of protected species are wildlife products, and much of what people call a fossil can fall under antiquities rules. If you see either offered, it should not be there. Prohibited items.
What protection covers
A specimen materially smaller than shown, an undeclared treatment, a different species, damage not shown: seven days from delivery.
Questions people ask
Is dyed agate a fake?
No, it is treated — and it should be sold as treated.
Should I ask about light sensitivity?
Yes, for anything coloured. Some minerals fade permanently.
Is a repaired specimen worth bidding on?
Yes, declared, at the right price.
More: buying minerals, storing them.