Flowstone

Flowstone was a substance made in the reactor core of the Stronghold and used by the Phyrexians to increase the mass of Rath and therefore allow the plane to overlay onto Dominaria. Flowstone was made from billions of nanorobots that could, through magic, be manipulated into virtually any shape and appear as almost any other material, making it incredibly useful. The ability to control flowstone was one of the main skills of the Rathi Evincars.

Following the Rathi overlay, there are presumably large amounts of flowstone left across Dominaria.