The Chemistry of Gemstones

Gemstones are fascinating in that they are an area of chemistry that really exemplifies how “pretty” chemistry can be to the general population. In general, gemstones can be measured on a variety of scales, illustrating properties such as color or hardness. Hardness for example is measured on the mohs scale, ranging from 1-10, with 10 being the hardest. Color of gemstones can be varied based upon the differences in chemical and atomic structure. These variations lead to the absorption of different wavelengths of light and can also yield slight nuances in the color of gemstones.

One common gemstone is the emerald, Be3Al2(SiO3)6, which gives rise to a vivid green color and has a hardness of 7.5-8.0 mohs. The green coloring is caused by chromium ions that replace aluminum in some of the locations in the structure. On the other hand, peridot, Mg2SiO4, is a gemstone that is green colored, but it does not have any chromium ions. Instead, the color is caused by Fe2+ ions that replace the magnesium ions in some locations. Another color comparison that is familiar to many is the difference between sapphire, aquamarine, and turquoise. These gems range from a deep rich blue to a lighter and sky blue. Though they are all shades of blue, each coloring is caused by a different ion, with titanium and iron ions replacing aluminum ions in sapphire, Fe2+ or Fe3+ ions replacing the aluminum ions in aquamarine, and copper ions coordinated to the hydroxide ions and water in turquoise. The differences in chemical structure can create a plethora of unique gemstones.

Perhaps one of the most famous gemstones is the diamond. It achieves the highest score on the mohs scale with a 10, and the coloring is generally clear. Most people know that diamonds are made from compressing carbon at a very high pressure. Not as many know about the interesting variation in diamond color and its effect on its value. For example, nitrogen is the most common impurity that is found in diamonds, and it yields a yellow or brownish color in the gemstone. These diamonds are considered to be of lower grade and quality. On the other hand, blue or pink diamonds are more valuable and sell for a higher price. The blue color in diamonds is caused by a boron impurity. The pink hue, as well as the rarest red hue, is caused by plastic deformation of the diamond crystal lattice. Yet another source of color is irradiation by alpha particles, which yields a greenish color in the gem.

Overall, it is fairly surprising that such a little amount of impurity can affect the value of many gemstones, and even more so that some impurities can raise the price of the gemstone. Since impurities in chemistry are generally considered to be bad, it is fascinating to see the impact that our consumer-based society has on a compound that is just a product of its crystal lattice and mineral impurities.