Everyone has forgotten them already. Way before Steven Universe, over at least 6,000 years ago, there existed more than four diamonds. White Diamond was still considered the highest, of course, since she had brought all of the diamonds besides Yellow, Blue, and Pink into existence, and Pink, was still the littlest. Black Diamond is a middle-class diamond; she still didn't have a proper court, army, colony, or even her own pearl, even though the rest of her class had all of the above, except for a colony and an army, only a few selected diamonds got to have that honor. Black sees some things she shouldn't, and her fellow diamonds beg her not to snitch, even if the consequences could be grave if White finds out. Black cannot decide whether to betray the diamonds below and above her to get where she needs to be or join in the mayhem, proving her loyalty to her diamonds knowing that even they could tell White.