Pristine Black Diamond

Pristine Black Diamond is a rare quest item and a mineral.

Source
Pristine Black Diamond drops off high-level Elite mobs throughout the world and in instances.

It is primarily found in Stratholme and the Scholomance. Other locations that have a lower drop percentile are Dire Maul, Blackwing Lair, and Blackrock Spire.

Other information
Currently, the primary demand for this item comes from players attempting to complete the achievement, which among other things requires a character turn in about 84 librams of,  and  for Shen'dralar reputation.

Several sources have noted that it may take 3-5 full clears of an instance (Scholomance, Stratholme, Dire Maul North/West) for each pristine black diamond obtained.

While the adventurer prefers to loot this item from a dead foe after a hours worth of killing, the entrepreneur might choose the time saver (although not a gold saver) and check the auction house.