This is the 15036th most frequent German word.
seltenes
"Seltenes" means "something rare" or "rare thing" in English.
The word 'seltenes' is used as an adjective describing the rarity of the book.
Das ist ein seltenes Buch, das nur schwer zu finden ist.
That is a rare book that is hard to find.
In this case, 'seltene' (plural form) is used to describe the artifacts as unusual or not common.
Dieses Museum enthält viele seltene Artefakte.
This museum contains many rare artifacts.
Here, 'seltenes' is used to characterize the phenomenon as uncommon or unusual.