Photographs of minerals mislead about size and colour, and prices built on photographs mislead for the same reasons.
What moves the number
Species, named as precisely as you honestly can.
Locality, which to collectors often matters more than size.
Crystal form and whether terminations are intact.
Treatment — dyed, heated, stabilised, coated — all normal, all declarable.
Matrix, where a crystal still sits on the rock it grew in.
Why two of the same differ
One has a locality and a species. The other has a photograph and the word "crystal". Collectors pay for data. Describing minerals.
How to find out rather than guess
Compare completed sales of the same species from the same locality in similar size and condition. Where you cannot identify either, describe what you can measure — weight, dimensions, colour in daylight — and let the room identify it. It often will.
Weigh and measure before you compare anything.
Two markets, not one
Collectors pay for species, locality and form. Decorative buyers pay for size, colour and how it looks on a shelf. The same piece can be a good buy for one and an expensive mistake for the other, so give the data either way.
What not to price on
What a stone is said to do for a person. That is a claim nobody can support, and it does not belong in the number or in the listing.
Questions people ask
Does dyeing destroy value?
It changes what the piece is. Declared, it sells. Undeclared, it is a dispute.
Is a bigger piece worth more?
Not necessarily. A small, sharp, well-localised specimen can beat a large dull one.
Can I sell a fossil?
Ask first — some fall under antiquities rules. Prohibited items.
More: pricing minerals, selling minerals.