Black, gray, and other similar dark-colored diamonds are referred to as "Black Diamonds" in the diamond industry. While black Diamonds are somewhat more common, red diamonds are the most rare.

Causes of Color
Diamonds typically consist of pure Carbon. However, traces of other elements are occasionally found in diamonds. These foreign elements can cause diamonds to assume different colors. Below is a listing of diamond colors and their causes:
yellow - nitrogen 
orange - nitrogen 
blue - boron 
green - irradiation 
red - unknown (possibly caused by structural anomalies) 
pink - unknown (possibly caused by structural anomalies) 
brown - uknown (possibly caused by structural anomalies) 
black - irradiation

Fancy color is a designation applied to stones with unusual colors. Fancy colors include darker yellows, browns, and grays, and very light to dark tones of blue, green, orange, pink, red and purple. Occasionally, black stones are cut as gems.

Photo luminescence (more commonly referred to as "fluorescence") refers to the diamond's appearance under ultra-violet (UV) light. (G.I.A. gemologists use a black-light to determine a diamond's fluorescence.) Many diamonds do not exhibit fluorescence when examined under UV light, but some do--ranging in intensity from faint to strong. Blue fluorescence is the most common, however yellow and white fluorescence also occur in some stones. Blue fluorescence is caused by traces of the element boron found in the diamond. The jury is out among gemologists as to wheter fluorescence adds to or detracts from a diamond's value. However, the Gemological Institute of America (G.I.A.) has recently published information indicating that blue fluorescence is usually considered a positive feature of a diamond.