To many people, a rock is just a rock. If anyone has been house shopping, you know how realtors will mention that the countertops are "granite" and you might wonder why, from house to house, they all look so different. Turns out (much to the chagrin of geologists in the housing market), those countertops are likely neither granite nor all the same type of rock. That's because rocks are made of different minerals and rocks get their names (mostly) from what minerals they contain.
Now, much of the time, once you've been trained to identify minerals, you can use those skills to give a rock its proper name. The countertop has quartz, plagioclase feldspar, potassium feldspar and biotite? That is likely granite. At another house it is all calcite...now that's marble.
