You donate them to your order. There has been a subject like this.

Please search before you post.

And just to clarify, I'm talking of another person's topic, not the double post you blatantly did.

alexanderwaring2 said:

new to the game how do you use diamonds

Hi! You use diamonds to creating technologies in the Arsenal. It's building in Order Citadel. You can find it in the bank or the order store.