Igneous, Sedimentary vs. Metamorphic Rocks: The main difference between Igneous, Sedimentary and Metamorphic rocks is the way that they are formed, and their various textures.Igneous Rocks: Igneous rocks are formed when magma (or molten rocks) cool down, and become solid. High temperatures inside the crust of the Earth cause rocks to melt, and this substance is known as magma. Magma is the molten material that erupts during a volcano. This substance cools down slowly, and causes mineralization to take place. Gradually, the size of the minerals increases until they are large enough to be visible to the naked eye.