When impurities are added to colorless Saphire Gemstones, brilliant colors are frequently produced. When chromium is added to colorless corundum, a red ruby is born, and a green emerald emerges when chromium is added to colorless beryl. Diamonds can be colored blue or yellow by the addition of impurities. The role of impurities in causing color is different here (see ). Through yet another mechanism (see ), the addition of impurities to corundum can produce a deep blue sapphire. Amethyst’s violet tint is the result of iron impurities in crystalline quartz, although this impurity acts differently than the chromium impurity that produces the red color of ruby (see ). Many Saphire Gemstones’ distinctive colors come from the presence of transition metals as impurities in an otherwise transparent crystal lattice. This can be because of a so-called crystal-field or, alternatively, a ligand-field effect. In this crystal field or ligand field effect, the field exerted by the host crystal on the guest impurity fixes the energy levels of the latter as a photon absorber. Put another way, the chemical bonding between the host crystal and guest impurity always involves the donation of electrons from the host crystal to empty energy levels in the metal impurity, binding the metal to the crystal.
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