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When visible white light enters a diamond, the stone absorbs some of the spectral bands, or wavelengths, and returns, or transmits, others to the viewer’s eye. This process of selective absorption determines the color of any material, including diamond.
The colors that return to the viewer’s eye and give an object its color are a combination of the wavelengths that aren’t absorbed. If little or no color is absorbed, all the spectral colors are returned, and the object appears colorless or white. If the entire spectrum is absorbed, the object appears black. There are an infinite number of possibilities in between.
Two conditions influence the way a diamond absorbs light and, in turn, determine its color. One is the presence of impurities, or atoms other than carbon. The other is the presence of structural defects like missing carbon atoms or distortions in the crystal lattice.
Structural defects that cause color are known as color centers. Many color centers are actually a combination of a structural defect and an impurity. This is because the defect creates an opening for the impurity, which is usually nitrogen or boron. Nitrogen is the most common impurity element in diamond. It’s also the most common cause of the yellow color. Boron causes shades of blue.
Distortions in the crystal lattice that are free of impurities can also act as color centers in diamonds. An example is the strain, or glide plane, that can develop along octahedral directions during crystal growth. It can cause light to be absorbed across the spectrum and make a diamond less transparent, with a smoky or brownish color.
Structural irregularities in a diamond crystal cause colored graining that can color the diamond. Graining causes the color in many pink and brown diamonds. www.gia.edu
