Doyouknow what sort of rocks are in the area you are collecting from? I have been studying geology at BSc level for a few years but to be honest its not always possibleto ID a rock at sight, you will sometimes need lab equipment and a microscope. But generally avoid limestone,anything with calcite looking veins, and sedimentary rocks like sandstone or conglomerates. Ideally rocks like slate, a metamorphic rock, or schists, and igneous rocks are best suited. I would take care boiling rocks. If you do this, make sure it is only for minutes at most. Rocks take a long time to heat up,but have a capacity to hold heat for a long time, also if they are cooled quickly they can explode spectacularly! Just be careful not to over do it.
Best if you can grab some good looking rocks,do the acid/vinegar test to see if they fizz, if they dont then take some pics post them on here and we'll see if they are ok.