It's a little bit like pornography ... hard to define but I know it when I see it.

The word "rare" (to me) as far as chips are concerned are NOT chips that routinely sell for $5 or $10 ... or $25 to anyone who wants one.

Rare; (to me) is a seldom offered chip that sells for hundreds ... or thousands of dollars ... just like "rare stamps", "rare coins", "rare baseball cars" or "rare paintings".

On the other hand, if I had a single personal chip made ... or painted a single painting, I guess one could consider that as as being "rare" ... but I don't think that it would pass the generally accepted "looks like, tastes like, smells like" universal rarity test.