The Spartans receive augments to strength and other natural capabilities. They're Mjolnir armor systems further enhance these. Natural stone isn't specially created to withstand getting hit by a train, and that's about as hard as these Spartans are hitting it. The Mjolnir armor plating as actually capable of deflecting small rounds. Covenant, and UNSC weapons don't exactly utilize [i]small[/i] rounds. The armor can only stop so many plasma shots before it weakens. Luckily the armor is self repairing. There is a reason for everything.