I found the name of a stone giant in Norse mythology named Hrungnir who was made out of stone. Are they a type of Jotunn or is this a singular instance?
Yeah. I made this mistake too. Despite how much Google generalizes it, he wasn't like the Stone Giants you know from D&D or LotR. Hrungnir wasn't crafted out of stone like Ullikummi (https://en.wikipedia.org/wiki/Ullikummi) or stone turned to flesh like Galatea (https://en.wikipedia.org/wiki/Galatea_(mythology)). It's more like Darth Vader with his cybernetics. He was a natural Jötunn who had certain body parts made of stone. Namely his shield, his head, and his heart (said to be a jagged, three-corned piece of stone). A lot of sources will say he was "Made of stone". This was likely an oversimplification of the translation. The writers of the story probably didn't feel like saying "containing a head, heart, and shield of stone" every time. So, they chose to use a phrase like "his body, consisting of stone", and the translations got it wrong. As far as I can tell, this was the only occurrence of something like this among their race.