That's a really interesting question...

Rare does not have anything to do with the number in existence nor should it have anything to do with value.

Chip collectors often use the word "rare" to define early or unusual chips but that isn't necessarily correct either.

If an auction said, "rarely seen for sale", that statement could be quite true for what is otherwise considered a fairly common chip.

What's all this mean? Hell, I don't know except to say that no chip currently being used on a table could possible be "rare" by my definition (unless you were cooking it I suppose)