Why such a difference is appraisal?
You show your onyx to the jeweler. He glances it over and says, "This looks to be worth around 75 gold."
From your best estimates, it looks like a piece of black onyx is treasure.
You think you could sell this to a jeweler.
You are certain that it is worth exactly 15 gold pieces.
The jeweler takes your onyx and hands you 75 gold for it.