Rocks are a mixture of minerals, where crystals and amorphous materials have a single ingredient. While not a gem material, granite is one of the most common and best known rocks. In the gem world lapis lazuli is the best known rock. Now we are getting into the heart of how gems are classified. The vast majority of gems are minerals.
Chemical makeup refers to the atoms the mineral is composed of.
Diamond has the simplest chemical makeup, with carbon being the only element present. Corundum is composed of just two elements, aluminum and oxygen. Its chemistry is expressed as Al2O3, meaning there are two aluminum atoms and three oxygen atoms in a molecule of corundum. The chemistry of other gems can get a lot more complicated. Molecular structure refers to how the molecules attach to each other.