Leystone is one of the Legion ores. It can be found in all zones of Legion.
Leystone is one of the shared mining nodes which means you can still mine the ore for a short time after someone else does. It is also useful for multiboxers, since all toons can mine the same node.
Leystone Basilisk has a chance to spawn when you mine a node, and will drop 9-11 Leystone Ore when killed.