2020- Black Diamond
Black Diamond is a gem used in some crafting professions and quests. It can be mined from Dark Iron Deposits and drops from mobs in Blackrock Depths.
As an ingredient
Black Diamond is used in the following recipes:
- Blacksmithing
[Darkrune Helm]- Leatherworking
[Mongoose Boots]- Jewelcrafting
[Figurine - Black Diamond Crab]
As a quest item
Black Diamond is needed for the following quests:
[55] Libram of Voracity- [55] Libram of Constitution
- [55] Libram of Tenacity
- [55] Libram of Rumination
- [55] Libram of Resilience
[55] Notes
Not to be confused with a 
[Pristine Black Diamond] which is also acquired as a drop. It is also used for quests, but despite the name, generally has a much lower value.
Black Diamonds are generally considered to be part of the Classic (pre-Burning Crusade) endgame, and thus less valuable with the Burning Crusade expansion.
Many classic crafted items using Black Diamonds are inferior to Uncommon or better items available in Outland, and thus available even to players without the expansion (via the Auction House). The [Darkrune Helm] may in theory be useful to people who do not play the expansion, yet are interested in the classic endgame, but could be replaced by a "of Shadow Protection" helm, or possibly any uncommon helm with +Stamina, from Outland, as well as certain classic boss drops like [Grand Crusader's Helm], Judgement Crown or 
[Helm of Wrath].
The Librams could be useful as low level enchants before other Augments become available, but a player heading to Outland at level 58 may find the price too high.
[Figurine - Black Diamond Crab] is a useful tank trinket, added with the expansion since it is a Jewelcrafted item. It is bind on pickup, limiting the use of Diamonds even in the expansion.
Prior to Patch 2.2, Enchant Cloak - Subtlety required Black Diamonds.
Patch 1.11 greatly increased the drop rate from mobs in BRD, and made Black Diamonds available from dark iron deposits.