What giant had a heart made of stone and fought Thor?

The giant described with a stone heart who fought Thor was Hrungnir.

Hrungnir is a jötunn, or giant, in Norse mythology. His confrontation with Thor begins when he races Odin from Jötunheim to Asgard. Odin rides Sleipnir, while Hrungnir rides his own horse, Gullfaxi. Although Sleipnir wins, Hrungnir enters Asgard, becomes intoxicated and threatens the gods. Thor is summoned to deal with him, and the two agree to a formal duel.

The Prose Edda describes Hrungnir’s head, heart and shield as made of stone. He fights with a whetstone against Thor’s hammer, Mjölnir. Thor kills him, but the giant’s enormous leg falls across Thor, leaving the god trapped until Thor’s young son Magni lifts it.

Magni receives Gullfaxi as a reward. That horse is an important source of confusion: Gullfaxi belongs to Hrungnir, while Sleipnir belongs to Odin. Geirröd and Hymir are other giants associated with Thor, but neither is the stone-bodied opponent in this duel.