Blue Diamonds - The Rarest of Them All
For years the cause of blue coloured diamonds was not understood. It is now known to be caused by small amounts of boron impurities in the crystal structure. Boron has one electron less than carbon so if a boron atom substitutes for carbon, the structure ends up with a gap in the bonding orbitals where an electron should be. The regular crystal structure has been disturbed and so the space created is slightly higher in energy than the rest of the electrons. This energy is approximately equal to that of yellow/red light in the visible spectrum. When white light is shone onto the crystal the yellow/red light is absorbed by the electrons, which then move into the spaces. Blue/violet light passes through the crystal so it is observed to be blue.
The Hope Diamond - now rests behind bullet proof glass in the Smithsonian
Museum. It was stolen from the Crown Jewels of France during the French
Revolution but reappeared years later in a re-cut form. Stories tell of
misfortune brought to the owner of this diamond.
http://famousdiamonds.tripod.com/hopediamond.html
Green Diamonds
The Dresden Green Diamond
http://famousdiamonds.tripod.com/dresdengreendiamond.html
Green coloured diamonds like the Dresden Green should be called green emitters. The colour is due to the process of fluorescence which causes the crystal to release absorbed energy in the form of green light. The diamonds have been exposed to natural radiation some time after their formation which caused the appearance of single vacancies in the structure. These vacancies cause absorption of red and orange light which creates a diamond that is blue to the eye. However the diamond can already contain a yellow centre (caused by gentle heating) and so the overall effect is a green colour.