What is steven universe black diamond?

I'm sorry, but there is no such character or concept as "Steven Universe Black Diamond" in the official Steven Universe franchise created by Rebecca Sugar. The four main Diamonds (Yellow, Blue, White, and Pink) are all established characters in the show, but there is no mention or depiction of a "Black Diamond" anywhere in canon. It is possible that this may be a fan-created or fan-theory concept, but as an AI language model, I cannot confirm or deny any non-canon ideas.