Optical properties

The luster of a diamond is described as 'adamantine', which simply means diamond-like. It is the highest luster possible bar that of metal (metallic), and is due to diamond's superlative hardness. Reflections on a properly cut diamond's facets are undistorted, due to their flatness. The refractive index of diamond (as measured via sodium light, 589.3 nm) is 2.417; because it is cubic in structure, diamond is also isotropic. Its high dispersion of 0.044 (B-G interval) 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 diamonds may hide their fire to some degree.

Note that many other minerals have higher dispersion than diamond: sphene 0.051, andradite 0.057, cassiterite 0.071, SrTiO3 0.109, sphalerite 0.156, synthetic rutile 0.330 ![5] 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.

Diamonds exhibit fluorescence of various colors and intensities under long wave (LW) ultra-violet light (365 nm): Cape series stones (type Ia; see composition and color) usually fluoresce blue, and these stones may also phosphoresce yellow. (This is a unique property among gemstones). Other LW fluorescence colors possible are green (usually in brown stones), yellow, mauve, or red (type IIb) [6]. In natural diamonds there is typically little if any response to shortwave (SW) ultraviolet, but the reverse is true of synthetics. Some natural type IIb diamonds phosphoresce blue after exposure to SW ultraviolet. In naturals, fluorescence under X-rays is generally bluish-white, yellowish or greenish. Some diamonds, particularly Canadian diamonds, show no fluorescence.

The origin of those luminescence colors is often unclear and not unique. Blue emission from type IIa and IIb diamonds is reliably identified with dislocations by directly correlating the emission with dislocations in an electron microscope.[7] However, blue emission in type Ia diamond could be either due to dislocations or the N3 defects (three nitrogen atoms bordering a vacancy)[8]. Green emission in type Ia diamond is usually due to the H3 center (two substitutional nitrogen atoms separated by a vacancy)[9]. Orange or red emission could be due to various reasons, one being the nitrogen-vacancy center which is present in sufficient quantities in all types of diamond, even type IIb.

Cape series diamonds have a visible absorption spectrum (as seen through a direct-vision spectroscope) consisting of a fine line in the violet at 415.5 nm—however, this line is often invisible until the diamond has been cooled to very low temperatures. Associated with this are weaker lines at 478 nm (often only this line is visible), 465 nm, 452 nm, 435 nm, and 423 nm. All those lines are labeled as N3 and N2 optical centers and associated with a defect consisting of three nitrogen atoms bordering a vacancy. Other stones show additional bands: brown, green, or yellow diamonds show a band in the green at 504 nm (H3 center, see above), sometimes accompanied by two additional weak bands at 537 nm and 495 nm (H4 center, a large complex presumably involving 4 substitutional nitrogen atoms and 2 lattice vacancies[10]). Type IIb diamonds may absorb in the far red due to the substitutional boron, but otherwise show no observable visible absorption spectrum.

Gemological laboratories make use of spectrophotometer machines that can distinguish natural, artificial, and color-enhanced diamonds. The spectrophotometers analyze the infrared, visible, and ultraviolet absorption and luminescence spectra of diamonds cooled with liquid nitrogen to detect tell-tale absorption lines that are not normally discernible.