Examples include labradorite and microcline feldspar. Optical Characteristics O ptical characteristics of gemstones are primarily derived from their chemical composition and physical structure. Color Color is the apparent result of selective absorption or transmission of different frequencies of visible light. Hue is a function of the frequency of light and is described by familiar terms such as red, orange, yellow, blue, green, indigo, and violet. Tone is a variation from very light to very dark.
Intensity is a measure of saturation, or purity, of a color.
The typical human eye can identify approximately 150 pure hues, but around one million colors. The differences among colors may be immediately obvious or so subtle that direct comparison under controlled conditions is required to discern them. Color acuity is also highly affected by fatigue, diet, and other factors, so it is unwise to attempt judging subtle color differences in gemstones such as diamond without comparison stones and careful control of lighting conditions. Pleochroism is the apparent change in color of a doubly refractive gemstone when viewed through different directions of the crystal structure. In most cases, the color variations are not obvious to the unaided eye and must be viewed through a polariscope or dichroscope, but in some cases, the pleochroic colors are strikingly obvious.