A mineral is a single non-living substance. Minerals are the building blocks that rocks are made of. Even though you cannot always see the different minerals in a rock, rocks are made from more than one mineral mixed together. It can be hard to tell if a specimen is a rock or a mineral; one important clue to deciding is color. Rocks may be one color or several colors, but minerals are always one color. Minerals may also have a crystal shape while rocks do not.