Chemical composition is like a recipe that describes the types and relative quantities of atoms in a given substance. A material’s chemical composition is expressed by a chemical formula that gives two kinds of information:
- The types of atoms that make up the gem
- The relative quantities of those atoms
The chemical formula for corundum (sapphire and ruby) is Al2O3 (Al = aluminum, O = oxygen). This means it’s basically made of aluminum and oxygen, and there are three oxygen atoms for every two aluminum atoms. Quartz is made of silicon and oxygen, with two oxygen atoms for every silicon atom, so its chemical formula is SiO2 (Si = silicon). Diamond has the simplest chemical formula of any gem—C—meaning it’s essentially pure carbon.
As you can see from the examples above, the numbers in a chemical formula show only the ratio of one element to another. The actual number of atoms in a gem depends on its size: There are many more atoms in a big stone than in a small one.
Nothing is absolutely pure, and every gem contains at least a few atoms that aren’t part of its normal chemical formula. These trace elements (sometimes called impurity elements) usually take the place of other atoms.
Color is one of the most obvious effects of a trace element. For example, corundum with very few trace elements is a colorless sapphire. But iron, titanium, or chromium often replace some of corundum’s aluminum. When this happens, and the trace elements are iron and titanium, the corundum is blue—a beautiful sapphire. When chromium is present, the corundum is red, which makes it a ruby.
