US Geologic Survey of a rough diamond in is host rock, showing the natural octahedral nature
What is a diamond?
A diamond is a solidified form of the periodic element 6, also known as Carbon. Yet this is a particularly beautiful form of Carbon due to its crystal structure which is also known as "diamond cubic".
Diamond sets the basis for the highest of highest hardness on the Mohs scale, and the highest of highest thermal conductivity of any material in nature.
Due to the arrangement of atoms in the diamond, very few elements can contaminate besides Boron and nitrogen which interestingly form colored diamonds from these so called "impurities"
Welcome to the world of "contaminants" and "impurities"
Examples of these "impurities" would be the "contamination" Boron in diamond which created the following:
Creative Commons Attribution-Share Alike 3.0 Unported
Now, how about the "contamination" of nitrogen?
These are but two examples of the infinite colors and varieties of diamond colors