Perception is the skill to use here
Your senses allow you to notice fine details and alert you to danger. Perception covers all five senses, including sight, hearing, touch, taste, and smell. [...] Perception is also used to notice fine details in the environment.
Your players are using their senses to find something not unlike a hidden door (albeit probably smaller) in this case.
As you say, appraise has the function
to determine the most valuable item visible in a treasure hoard.
Emphasis mine. Appraise has nothing to do with actually finding something that is hidden. This skill use means the character is just glancing over each and every single visible item in the hoard (which is not hidden at all), and roughly estimating its value.
I personally would ask for perception checks as well in case the most valuable object is hidden inside the hoard (such as a particularly large gemstone in a heap of jewelry, which is still relatively small).