They aren't unique, like platinum or titanium (6)

I believe the answer is:

alloys

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.