The Black Diamond is one of the first rare materials you'll encounter in Return to Moria, needing it as you do to repair the Mapstone in the first camp in order to unlock fast travel. While you can randomly get this material by fighting Orcs and Goblins during horde events—the first of which occurs when you repair the great forge to get the Steel Pickaxe—there is a more consistent method of gathering them.
You'll want a fair few Black Diamonds knocking around your inventory, since they are used to craft one of the best pickaxes, plus to repair and unlock the Mapstones you need to travel between your various camps. All that said, here's how to get Black Diamond in Return to Moria.
How to get Black Diamond
The best way to get Black Diamond is via Orc and Goblin enemies. While these have a very low chance of dropping Black Diamond as treasure when you defeat them during a horde attack, your guaranteed way of finding Black Diamond is by plundering Orkish Chests from the Mines of Moria area onwards. While these chests are also found in the Elven Quarter, none of them there seem to contain Black Diamond.
You can find Orkish chests in little enemy encampments and open them by smashing the totem in the centre of each camp to get an Orc Key that unlocks them. If you can find your way to Orc Town from the Mines of Moria area, there are a few totems and Orkish Chests there that all contain multiple Black Diamonds, so that's your best bet for getting lots early on.
Once you destroy a totem, all the enemies surrounding it will disappear when you return to the area, so you don't actually have to defeat them to open the chest. Just evade them while using a pickaxe to smash the totem, grab the key that drops, and then return once they've disappeared to open the chests. Since the regular Orc and Goblin enemies don't drop much of particular value, you don't need to worry about missing out.