Master jewelers would probably be able to identify "Hey, this isn't normal"
Wouldn't it be nuts if the items were just "A purple piece of amethyst" but in fact it was a small piece of purple diamond, but people without the skill couldn't tell, till it was worked?
> forage stones
You pick up a small, black stone.
> value stone
A small, black stone would seem to be an unworked black diamond.
A unworked black diamond would seem to cost about 550 obsidian pieces.
A unworked black diamond would seem to weigh 1 stone.
> forage stones
You pick up a small, black stone.
> value stone
A small, black stone would seem to cost 2 obsidian pieces.
A small, black stone would seem to weigh 1 stone.
Ability to properly assess what an item actually might be would be determined by your Guild and your level in the relevant crafting skill, such as Stone crafting or Jewelry crafting.
Don't have the ability to determine your stuff for value, yourself? Give that merchant in the Bazaar something to do. Bring your stuff and for a fee, they'll assess it for value and any hidden properties.