Blackstone is a precious gem found in the continent of Avaris. It appears shiny and black in colour, looking especially bright and is extremely rare in the modern age.
Blackstone mines were originally discovered near the area now occupied by the citadel of Pereshka. Heavy mining and many years have led to it being extremely rare, as another source of this precious gem has since never been discovered.
Blackstone has the curious property of suppressing magical energies. In fact, the original bed of blackstone mines was responsible for keeping magical enemies out of Avaris. The original mining of blackstone is what led to magical enemies permeating Avaris. The unfortunate side-effect was the breaking of the barrier that existed between this world and the shadow-lands.
The largest single collection of blackstone was perhaps found on the famous Spellbreaker. With Spellbreaker lost to time, it is anybody's guess where any of those precious gems are.
Blackstone is widely sought after in the current age due to having the ability of suppressing magic, which gives the wielder a large benefit in battle. However, blackstone is a banned substance within the empire. Personal possession of blackstone is only allowed by a decree from the ruling council, which is usually never done.
It has never been clear whether the empire still possesses blackstone in their vaults, but rumors circulate of large expeditions to find more sources of blackstone.
The secret 10th citadel of the empire, Shekara, has whispers of blackstone deposits nearby.