Notes on Rocks What is a rock? It is a hard substance formed from one or more minerals. What is a mineral? It is a naturally occurring, inorganic solid that has a definite chemical composition and crystal structure. Most minerals are made from the Eight most abundant elements in Earth’s crust: Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, and Magnesium. What are the types of rocks and how are they categorized? There are three types of rocks: Igneous, Sedimentary and Metamorphic. They are grouped into these categories by how the rocks form. The Rock Cycle