Now this is random, ik, but I've been having this idea about Black Diamond for the longest time. So here's my interpretation:
Now, I like to imagine that gem kind was made by another advanced species a long long time ago, with White Diamond being the first to be made, and the one to control the gems when necessary.
I got this idea because when I thought about it, it's could start out as just making sentient robot helpers.
Now White was not happy with being a servant to another species, especially one that is organic and inferior to her perfection, let alone that of the gems. The creators noticed it, and so made a brother for White, thus comes Black Diamond.
Black was made explicitly to be able to resist White's influence, and drain the energy of gems this actually 'killing' the gem, leaving it merely as stone.
To ensure that Black would be subservient, they experimented with having him be raised by their species rather than immediately be set to the task, in the idea that getting him emotionally attached to their species and view members of it as family would deter him from rebelling.
For the time, it worked wonders. White herself was pissed at the existence of another gem in an equal position to her, but eventually warmed up to him as family.
White still would wish to stop serving though, and so came up with a plan. With Black being tailor made to be able to stop her, she can only succeed by bringing him to her side.
To do that? She slowly orchestrated and manipulated their creators to kill Black's caretaker, feeding them ideas of how the caretaker had a hold over Black, and could use him against them anytime they wished.
Black of course was heartbroken as he learned that his caretaker died, and was furious how the murderer was unpunished. In his vulnerable state, he was convinced by White that they should rebel and free gem kind from such murderers and brutes.
Then followed a long war with the gems against the creators, eventually winning and wiping out the creator species.
When it came for interplanetary conquest and expanding, the two had differing approaches that brought conflict between them.
While White had the most efficient method of harvesting the planet till it died and there was almost nothing left of it, Black's softer nature reverted to conquest.
Black would take over the planet but spare it's species, extracting what they could for gems and stopping when the first signs of harm showed on the planet.
White hated that method due to how inefficient it was and eventually the both of them got into an argument. It ended with black cutting ties with homeworld and taking his gems and leaving.
White meanwhile began making the other diamonds out of spite, as if to show her brother he could be replaced when he eventually returned...except he didn't.
Thus Black is unheard of and not a thing to gemkind simply because he is no part of the diamond authority, and stopped being part of homeworld before the bulk of modern gems even came to be.
Now I imagine him being male because the idea of all gems being made feminine is because White doesn't wish to be reminded of her brother made my brain tingle and so I ran with it . I imagine him to be of equal size to White and for his style to be some weird blend corporate suit and the clothes those warriors wear in Wuxia films, with very long yet spikey hair.
Unfortunately I don't have the artistic skills to draw that so enjoy coming up with that image, and thank you for taking the time to read this random ramble of an idea that was stuck in my head for 2 years.