There isn't really much you can do to make a rock "aquarium safe".
Depending on the minerals making up the rock, it will either be able to dissolve and raise the pH and hardness, or it won't.
If you are doing a marine or rift lake cichlid tank, it probably won't matter much. other then that, it depends on the minerals in the rock, how touchy your tank's water parameters are, and how often/how big your water changes are.
I've got no clue what those are based on the pics.
You could try an acid test, if it fizzes, it will definitely dissolve some in the tank. You could also try scratching it with a steel nail/knife point. if some of the steel rubs off on the rock, there is a good chance it's some sort of silicate-based mineral, and probably won't affect water chemistry much. But that's far from reliable, and a gross generalization, etc.
If you are really set on having a given rock, and it's either unsafe, or you are really concerned about it, you could try coating it in epoxy.