There are three different types of rocks and their unique ways of being formed are what led to how they are different now. Sedimentary rocks are formed by layers of soft sediment slowly combining to become things like sandstone, limestone, shale, etc. Igneous rocks are formed from cooling magma. If the magma is cooled quickly there are small crystals, but if it is cooled slowly there are larger crystals. Obsidian, granite, and basalt are all igneous. Metamorphic rocks are formed when already existing rocks are put under intense heat and pressure, and they typically have ribbon-like layers and crystals.