technique and its duration, and in some cases the diamond may be left radioactive to some degree. Some irradiated diamonds are completely natural; one famous example is the Dresden Green Diamond. In these natural stones the color is imparted by "radiation burns" (natural irradiation by alpha particles originating from uranium ore) in the form of small patches, usually only micrometers deep. Additionally, Type IIa diamonds can have their structural deformations "repaired" via a high-pressure high-temperature (HPHT) process, removing much or all of the diamond's color. Luster The luster of a diamond is described as 'adamantine', which simply means diamond-like. Reflections on a properly cut diamond's facets are undistorted, due to their flatness. The refractive index of diamond (as measured via sodium light, ) is 2.417. Because it is cubic in structure, diamond is also isotropic. Its high dispersion of 0.044 (variation of refractive index across the visible spectrum) manifests in the perceptible fire of cut diamonds. This fire—flashes of prismatic colors seen in transparent stones—is perhaps diamond's most important optical property from a jewelry perspective. The prominence or amount of fire seen in a stone is heavily influenced by the choice of diamond cut and its associated proportions (particularly crown height), although the body color of fancy (i.e., unusual) diamonds may hide their fire to some degree. More than 20 other minerals have higher dispersion (that is difference in refractive index for blue and red light) than diamond, such as titanite 0.051, andradite 0.057, cassiterite 0.071, strontium titanate 0.109, sphalerite 0.156, synthetic rutile 0.330, cinnabar 0.4, etc. (see dispersion). However, the combination of dispersion with extreme hardness, wear and chemical resistivity, as well as clever marketing, determines the exceptional value of diamond as a gemstone. Fluorescence Diamonds exhibit fluorescence, that is, they emit light of various colors and intensities under long-wave ultraviolet light (365 nm): Cape series stones (type Ia) usually fluoresce blue, and these stones may also phosphoresce yellow, a unique property among gemstones. Other possible long-wave fluorescence colors are green (usually in brown stones), yellow, mauve, or red (in type IIb diamonds). In natural diamonds, there is typically little if any response to short-wave ultraviolet, but the reverse is true of synthetic diamonds. Some natural type IIb diamonds phosphoresce blue after exposure to short-wave ultraviolet. In natural diamonds, fluorescence under X-rays is generally bluish-white, yellowish or greenish. Some diamonds, particularly Canadian diamonds, show no fluorescence. The origin of