A rock is a naturally occurring solid mixture of one or more minerals or organic matter. A mineral is an inorganic solid that has a definite crystalline structure, and it is naturally formed. There are different elements that make up a crystalline structure. Elements are pure substances that cannot be broken down into simpler substances by ordinary chemical means. All minerals should contain at least one or more of the 92 natural elements. A compound is two or more elements that have been chemically joined together.