Eijiro Kirishima's rival, Tetsutetsu Tetsutetsu hardens his body into steel. Eijiro seems to be very similar to him, but I dont remember it was mentioned which does he harden his body into. What is it?
Possibly, just hardened skin (and body as well), but as hard as steel as his quirk is comparable with Tetsutetsu steel body. Here we see his hardening quirk allowed him to hardening his body, but still we see it with skin color
In addition to his hardened skin, his whole body seems to be able to hardened (also changed its shape - see picture) as well, as stated in wiki:
The ability not only protects Eijiro from physical attacks, but also from heat and shock waves. This Quirk is strong enough to protect its user from several tons of metal falling on him as well as multiple explosions through an extended amount of time