You might think of a rock as just being cool to climb or throw into a pond, but to a geologist the word has a very special meaning. A rock is a naturally occurring solid substance that is usually made of minerals. Some rocks are made from only a single mineral but most are made from several different minerals that have joined together. By identifying the different minerals found in a rock and by understanding the way that the minerals formed, geologists place rocks into one of three different groups. These three major groups are known as igneous, metamorphic, and sedimentary rocks.