Photographs of minerals mislead about two things above all: size and colour. Both are fixable in a live room.
Ask for scale
"Can you put it on a scale?" and "Can you hold something ordinary next to it?" A coin, a hand, a ruler. Specimens photographed alone look twice the size they are, and this is the most common disappointment in the category.
Ask the three questions
"What species is it?" — and how confident the seller is. "Where is it from?" — locality matters to collectors. "Has it been treated?" — dyed, heated, stabilised, coated, reconstituted. All normal, all should be declared.
A seller who answers "sold to me as X, I cannot confirm it" is being straight with you. One who is certain about everything, with nothing to show, is not necessarily.
Ask for daylight
Colour under a show light is not colour. Ask for the piece in daylight or near a window, and ask to see it turned.
On health claims
If a seller tells you a stone will treat, cure or protect against something, treat that as a sales line and not a fact. Buy the stone because you want the stone.
Two things that cannot be sold here
Coral, ivory and the shells of protected species are wildlife products. Much of what is loosely called a fossil can fall under antiquities rules. If you see either being offered, it should not be there — report it, and see prohibited items.
What protection covers
A specimen materially smaller than shown, an undeclared treatment, a different species than described, or damage not shown: not as described, seven days from delivery.
Questions people ask
Is dyed agate a fake?
No, it is treated — and it should be sold as treated. The problem is silence, not the dye.
Does locality really matter?
To collectors, often more than size or colour. To a decorative buyer, not at all. Know which you are.
How do I judge colour in a show?
Daylight, turned slowly, and ask for a second look if the first was under a lamp.