Blackstone, known in High Gothic as noctilith, is a mysterious, black-hued, stone-like substance of unknown provenance that possesses the ability under certain conditions to nullify outright psychic energies or to absorb them and unleash them with spectacular force. Though relatively rare, it is found in concentrations on certain worlds across the galaxy, particularly those planets that are Tomb Worlds of the ancient Necron Empire.
Blackstone was used by the Necrons during the War in Heaven to construct special, pylon-like devices on various worlds across the galaxy that were capable of creating tetrahedral regions of space-time immune to incursions from the Immaterium.
Blackstone's true purpose and properties remained unknown throughout Imperial history, and only at the close of the 41st Millennium did its true import become clear. Perhaps, if the Tech-priests of Mars had come to understand its significance earlier, the opening of the Great Rift could have been prevented, and the galaxy kept from plunging further toward utter calamity.
Blackstone is mined from a smooth dark rock, similar to obsidian or onyx in appearance. It registers all manner of pecularities when surveyed by Omnispex or prognost wave, and has baffled every cryptogeologist and terrastitian who has examined it. The substance is found only on certain mineral-rich planets across the galaxy, though a close correlation has been found between worlds settled by the original Mechanicum -- the forerunners of the Adeptus Mechanicus -- and the "Quarry Worlds" upon which the mineral is present. Some of these worlds have clearly been mined specifically for this subtance, or possibly had large monoliths of blackstone conveyed to their surface and installed there by forgotten agents from before the dawn of recorded human history.
Since the opening of the Great Rift, the Tech-priests of the Adeptus Mechanicus have become aware of the true properties of blackstone and have sought to mine it in large quantities wherever possible. Under the direction of Archmagos Dominus Belisarius Cawl, the Mechanicus has begun blackstone mining operations across the galaxy. This is part of an attempt to gather enough of the material for the Imperium of Man to construct its own zones of realspace stability to contain the Chaos incursions facilitated by the Great Rift. However, this quest has brought the Mechanicus into conflict with the various dynasties of the Necrons, who are also seeking to gather the resource as they did in ancient days. Like Mankind, the Necrons intend to use the blackstone to close the Great Rift and retake the galaxy for their own.