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 this absorption, now known as "N3", by the Indian school under Sir C. V. Raman in the 1930s and 1940s led to a basic understanding of the system, which they observed in both absorption and luminescence. The N3 center is a structural defect in the diamond, and the absorption of light occurs by exciting electrons in this defect from one well-defined energy state to another. When the electron returns to the original energy level, luminescence is produced. Detailed studies of natural diamonds over the subsequent 60 years have discovered large numbers of absorption and emission lines, characteristic of different defects. In 1904 Sir William Crookes showed that a colorless diamond could be turned green by long exposure to radium salts.