WARNING: Before you get started
Here's the downside of this technique. You need to be careful. For rock boiling and any other uses of hot stones, make sure you gather the rocks from a high and dry location. Waterlogged rocks can explode violently when they heat up in a fire and send sharp stone flakes flying in all directions. The steam builds pressure in the rock causing it to blow up like a grenade. In addition to waterlogged stones, you'll want to avoid slate, shale, quartz and obsidian as they are prone to explosion regardless of their location near or away from water. If uncertain about the types of stone you have found, toss some samples into the center of a large fire and go 50 yards away. Wait 20 minutes at that distance, watching and listening to the fire. If the rocks are going to explode, they'll pop in the first 15 minutes. Some rocks may just crumble into sand, and these aren't good for rock boiling either. Examine the rocks after the fire has died out, and any that remain intact are a good type of rock for boiling and heating.