User blog comment:Dragonzzilla/Skyrim Suggestions - Giantsore/@comment-4808313-20120210185327

I could be mistaken, but I think those blackish-rocks around Giants' Camps are just average boulders that are painted/decorated by the Giants. I assumed they were put there to show a kind of rudimentary culture developing amongst the Giants, thus making them more human/relatable and less like Trolls or other 'mindless monsters and beasts' that you can encounter in-game.

I could be way off though. I admit I never really get close and linger long enough to get a good look at the stones and I'm pretty much guessing.

I think if you or someone were going to make a mod around this concept it'd be really interesting to change those decorative into a type of 'new metal', regardless of what they are now. Perhaps they could be pieces of a meteorite that the Giants consider to be holy, or something along those lines.