Color gives both distinctiveness and character to a gemstone. May gemstones appear colored because either part of the white light traveling through them is absorbed within the mineral structure or reflecting the light within the gemstone. This absorption of the light is generally due to the presence of particular chemical elements and damage or irregularities in the crystal structure. Most of the gems are colored due to presence of chromium, iron, manganese, titanium, and copper. For instance, chromium gives the intense red of ruby and brilliant greens of emerald. Copper gives the blue and greens of turquoise and malachite. In colorless and cut gemstones, one can occasionally observe a play of color, which comes about through dispersion. When light enters a gemstone the various wavelengths are differently refracted, red the least and violate the most, so that the color spectrum is spread out. Gem minerals vary greatly in their ability to disperse light therefore different colors with different qualities are observed in gemstones. Color dispersion is especially high in diamonds where it produces a beautiful play of color that is called fire. Play of color in opal gemstone happens due to interference phenomenon. Interference happens when light falls on very thin, transparent mineral layers, it is reflected from both the upper and lower surfaces. Since the reflected rays have traveled different distances the wave troughs and peaks of the various wavelengths either coincide or are out of step. A color is enhanced if they coincide but little or no color is seen for out of step wavelengths.Gemstone color chart can be found here.