The Chemistry of Colored Diamonds

Carbon has two major allotropic forms: graphite which is the thermodynamic stable form at 1 atmosphere pressure and diamond which is the stable form under very high pressure conditions. Thus at 120-150 km below the earth crust at 2-3 MAt pressure and ~1,200C, diamonds are the basic form of carbon.

Fortunately, when diamonds are ejected to earth surface via volcanic eruptions, if the diamond cools down fast, the slow kinetics of oxidation to carbon dioxide or transformation to graphite keeps the gems to survive as diamonds at 1 At pressure.

Most diamonds are colorless with a very high refraction index of 2.43. However colored diamonds (defined in the jewelry industry as Fancy Diamonds) are known, though these gems are rare and much more expensive than regular diamonds. The main source for color in diamonds is the presence of small concentrations of nitrogen atoms (<100ppm) in the crystal structure, though in few cases boron can induce some blue grey color. Nitrogen can occur in 10-15 forms in the crystal structure and every form causes different optical properties of the diamond. Thus red, blue, yellow, green and almost every other color is known.

The presentation will discuss what nitrogen structures are responsible for any color and how chemical treatments (e.g. high temperature, pressure, radiation etc.) can change the nitrogen form of the N contaminated diamonds to the desired color and what analytical and chemical techniques are used for the process. Also samples of different colored diamonds will be presented.

Keywords: diamonds, colored diamonds, chemical treatments of diamonds, HPHT process