Hydrochloric acid dissolves only the calcite. Calcite has a hardness of 3, and quartz 7, so you could see which scratched the other, the quartz will not be scratched by the other. The same volume of quartz is not as heavy as the same volume of calcite. Specific gravities are 2.6 to 2.65, and 2.71 respectively, so that might be a close one if you have small samples or imprecise equipment.
