Minerals are usually crystalline structures that contain a regular repeating array of atoms and molecules. The different colors of precious gems often arise due to a defect in the crystal lattice. For example the mineral corundum has the formula Al₂O₃, however, if trace amounts of chromium are present the crystal takes on a red hue and is called ruby. If it contains a trace element of iron, titanium, chromium, magnesium, or copper the crystal is called a sapphire and can occur in a wide range of colors. While these impurities result in aesthetically pleasing minerals crystals grown for research need to be a pure and as free of defects as possible. One way to improve the quality of lab grown crystals is to slow their growth rate. You will identify a mixture of metal cations and nonmetal anions that can produce a crystal of a single desired color.