The colors we see depend on how light travels through a gemstone. Crystal structure and the metallic elements present in minute amounts in the gemstone determine this. Many gems appear colored because part of the white light travelling through them is absorbed within the mineral structure. The causes of absorption are complex, generally involving the presence of particular chemical elements and damage or irregularities in the crystal structure.
A limited range of metals colors most gems. The most important of which are chromium, iron, manganese, titanium, and copper. Chromium gives the intense red of ruby and brilliant greens of emerald and demantoid garnet. Iron causes the more subtle reds, blues, greens, and yellow in almandine garnet, spinels, sapphires, peridots, and chrysoberyls. The most prized blue sapphires are colored by titanium with iron. Copper gives the blues and greens of turquoise and malachite. Manganese gives the pink of rhodonite and orange of spessartine garnet.
In most gems these metallic elements occur as impurities usually in minute amounts. Such gems can show a wide range of different colors and because they contain such small amounts of impurity the color of some may be altered, enhanced or destroyed by heating or by irradiation with gamma rays and high energy sub atomic particles.
In a few gems the coloring elements form an essential part of the chemical composition for example the copper in turquoise, manganese in rhodonite, and iron in peridot and almandine garnet. These gems have a very limited color range, generally restricted to shades of one color. Such colors are stable and impossible to alter greatly without destroying the mineral.
Crystal structure affects the way in which light travels through a substance. In all minerals other than cubic and non-crystalline minerals, light entering the mineral is split into two rays that travel at different speeds and along different paths through the crystal structure.
In colored minerals the rays may be differently absorbed within the crystal structure and emerge as two or three different colors or shades of the same color. This effect is called pleochroism and can be particularly helpful in identifying gemstones.
Pleochroism causes the directional variations in color seen in many gem minerals. Therefore, a gemstone looks a different color when turned and looked at from different directions. Viewing these different colors is made easier with the use of a small instrument called a dichroscope. A dichroscope enables two colors to be seen at the same time through the eyepiece while turning the gemstone. Dichroic gemstones have two colors and trochroic gemstones have three different colors or shades of color when viewed from different directions.
The brilliant colors of opal and diamonds arise when white light is split into its constituent colors, the colors of the rainbow. White light consists of electromagnetic waves of different wavelengths, each wavelength appearing a particular color.
Dispersion is the origin of fire in gemstones. The fire of a gemstone is seen as flashes of the colors of the rainbow as the gemstone or light source are moved. When light enters a mineral the various wavelengths are differently refracted, red the least and violet the most, so that the color spectrum is spread out. Gem minerals vary greatly in their ability to disperse light. The dispersion can be measured as the numerical difference between the refractive indices of specific blue and red wavelengths.
Interference causes the iridescence in labradorite and the rainbow effects seen in cleavage cracks and on tarnished surfaces. When light falls on very thin 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 litter or no color is seen for out of step wavelengths.
In opal, which is composed of transparent regularly sized and stacked spheres, light is scattered by the network of spaces between the spheres. Interference occurs between the emerging rays, the range of colors seen depending on the size of the sphere and the angle at which the opal is viewed. Larger spheres produce a complete spectrum as the opal is tilted, but small spheres generate only blues and violets.
There's much more at Gems Explained than information on colored gems .