Stone is the base material that asteroids are made of and is the most abundant resource per volume in the game. Like all ores, the size and mass of a chunk is the result of how much ore is there. It can be processed in a Refinery to produce Gravel at a 90% ratio. Which is to say 10 kg of stone will produce 9 kg of gravel. Gravel is used only in reactor components.
Stone in an asteroid, and a closeup of a mined stone.