Hello there Mr. Kill_spree:
To get the diamond reward for the 10 diamond characters, you need 10 diferent diamond characters for it, not only diferent variants.
Let´s say you have Frezze frame + dream catcher (Both Peacocks) + 8 other diamonds. The game only counts 9 characters there.
But if you have a diamond 1 Filia + 1 big band + 1 beowulf + 1 fukua + 1eliza + Parasoul + 1 Annie + 1 Umbrella + 1 Miss Fortune, and you get from a relic a diamond Squigly OR evolve one to dia, then the archivement unlocks and you get a brand new Diamond Relic!
Hope this info helps!!