Composition and Color of Diamonds
Type I diamonds have nitrogen atoms as the main impurity. If they are in clusters they do not affect the diamond's color
Type II diamonds have no nitrogen impurities. Rarely, they contain no other impurities: these are Type IIa, colored pink, red or brown by structural anomalies arising through plastic deformation. Type IIb are the natural blue diamonds which contain scattered boron within the crystal matrix.
Diamonds occur in a variety of colors - steel, white, blue, yellow, orange, red, green, pink, brown and black. Colored diamonds contain impurities or molecular defects that cause the coloration, whilst pure diamonds are always transparent and colorless.
In the late 18th century, diamonds were demonstrated to be made of carbon by the rather expensive experiment of igniting a diamond (by means of a burning-glass) in an oxygen atmosphere and showing that carbonic acid gas (carbon dioxide) was the product of the combustion. The fact that diamonds are combustible bears further examination because it is related to an interesting fact about diamonds. Diamonds are carbon crystals that form deep within the Earth under high temperatures and extreme pressures. At surface air pressure (one atmosphere), diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (δH = -2 kJ / mole). So, despite De Beers' ad campaign, diamonds are definitely not forever. However, owing to a very large kinetic energy barrier, diamonds are metastable; they will not decay into graphite under normal conditions.
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