Boiling rocks is perfectly safe, they won't explode.
That's a myth that keeps getting passed around on a "better safe then sorry" basis.
And, while it's hard to tell from pictures, those rocks look like some sort of silicate, and I would imagine they are safe.
Try scratching the rock with a piece of steel (a nail, knife point, awl, etc.) if you can't scratch the rock, it's safe. If you can scratch the rock, it might or might not affect hardness/pH. As PlantedRich pointed out, if your water is already hard, and you aren't using RO/DI, it's probably fine. Otherwise, try the acid test on the scratched up portion, or take two containers of water, put the rocks in one, and test (Hardness, pH) each one at the beginning, and then a week or two later.
Avoid really spectacularly colored rocks, and metallic colored rocks, these are often metal ores that can cause problems.