A rock is a consolidated mixture of the same or different minerals. By consolidated, we mean hard and strong; real rocks don’t fall apart in your hands! A mixture of minerals implies the presence of more than one mineral grain, but not necessarily more than one type of mineral. A rock can be composed of only one type of mineral (e.g., limestone is commonly made up of only calcite), but most rocks are composed of several different minerals. A rock can also include non-minerals, such as fossils or the organic matter within a coal bed or in some types of mudstone.
Thumbnail: Half Dome in Yosemite is a classic granite (a common intrusive igneous rock) dome and popular rock climbing destination (Public Domain; Jon Sullivan).