The unique properties of diamond are responsible for its pre-eminence as a gemstone, and give it a glamour and attraction unprecedented for any other mineral. As the first member of group IV of the periodic table of elements, carbon, in its crystalline form as diamond, has also fascinated scientists for at least 300 years. Many experimental techniques have been employed in the study of diamond, and of these, optical spectroscopy has proven one of the most fruitful. The absorption line at 415 nm, characteristic of "Cape Yellow" diamonds, was first documented by Walter in 1891. Further work on...