What Does Very Rare Mean

1 : seldom occurring or found : uncommon. 2a : marked by unusual quality, merit, or appeal : distinctive. b : superlative or extreme of its kind.

What is very rare?

Something that is rare is not common and is therefore interesting or valuable. […]

Can we use very rare?

Since adverbs like rarely cannot function as subject complements, but adjectives like rare can, very rare is the only possibility here.