We are differentiating between metamorphic and sedimentary rocks. There are three major types of rock: igneous, metamorphic, and sedimentary. Igneous rocks are formed by the cooling of hot lava. Sedimentary rocks are formed by the depositing of various sediments in different layers on the Earth's surface or under a body of water. Metamorphic rocks are caused by existing rocks of any type being altered through heat and pressure. In terms of observable differences between sedimentary and metamorphic rocks, metamorphic rocks often contain layering called foliation that is irregularly shaped due to changes in crystallization of the minerals, while sedimentary rocks have more evenly distributed layers. Also, since metamorphic rocks are formed through recrystallization of minerals at high heat and temperature they are very dense and strong whereas sedimentary rocks are more granular with pores and are "softer", or at least more easily breakable with tools.