They aren't unique, like platinum or titanium (6)
I believe the answer is:
alloys
'they' is the definition.
Both the definition and answer are plural nouns.
Perhaps there's an association between them I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for alloys that I've seen before include "Blends of metals" , "Tempers" , "Puts down" , "Metals formed from mixtures of other metals" .)