The Black Diamond was used as primary monetary system in ancient ages, but as the population grew there was a need for something more abundant, gold. Nowadays this dark yet strangely shiny gemstone is used amongst high-end merchants and artisans. The name "Black Diamond" obviously describes its physical properties, but many dispute that it's a diamond, some say it is actually hematite, hardened by a dragons breath.
You might be able to scavenge some Black Diamonds from patches of dirt where thieves hide their spoils, or you could slay some monsters and hope they happened to have slayed an unlucky adventurer who was carrying some. There's also word about a mysterious benefactor gives out some Diamonds to those who achieve a noteable level (multiple of 100, see Globals) and sometimes when searching for a monster, the monster flees and you recieve the Black Diamond he was carrying.
Reward Boxes are known to sometimes contain a secret compartment with a Black Diamond in it.
You could also bribe the god of this world to summon a pile of diamonds for you. But you'll need some otherworldly form of money... weird coins and pieces of paper. Strange gods. (AKA IRL-money, you silly. For the prices, check the 'Cash Shop' button ingame!)
Many a merchant accepts Black Diamonds as a form of payment. You can spend them in the Emporium or you could increase your chance of enhancing your weapon by ... "motivating" the artisan. With diamonds.