I can buy these for 0 gold. So I just bought 99 and it didn't cost me one penny. Not sure if its relevant, but I'm in the gold creed and buying from a gold merchant.

You can find one after the Siam Lake Warp. Go right from the Sanctuary until you reach the a door with a lever behind it, jump the ledge and there's one right there.

You cannot get this from the creed, Edited page.

my bad, you can get the diamond cluster from this creed but ONLY after increasing your devotion.

just bougt 100.. tought it was a bug

In the current version of the game, upon reaching rank 4 in the creed of the house of splendor, the vendor of said creed will sell diamond clusters for 0g. obviously a glitch, but the earliest you can obtain diamond clusters and really helpful for poleaxe builds, given how far in the game you must usually get before upgrading from the basic infantry pollaxe

Way is it ner the end of the game

I thought it was a mineral, but instead you're pretty much improving your weapon with very expensive eldritch caviar?