In school we did an experiment with the one you said is quartz. We mixed pieces of it with some acid to see if it was carbonate, and indeed it was, but when I look up online quartz seems to be made of SiO4. I don't think it will produce the same results in this experiment, so it's most likely something else.
Fri 4 Aug, 2017 01:50 pm
The first rock seems to be made by grey limestone with a pactch of calcite crystals. It could also be dolomite, but as you say that it easily reacts to acid, I would go for calcite.