corundum, color.4, gems

The expanding role of recording spectrophotometers in gem identification is illustrated through the development of the Gem Testing Laboratory of Great Britain’s equipment and several diagnostic case studies. Although the hand-held spectroscope remains valuable, its limitations become significant when precise wavelength measurements, absorption-band shapes, or information outside the visible region are needed to distinguish synthetic gems and treated natural material. Diamond absorption features provide an example: the H3 zero-phonon line at 503.2 nm and the H4 line at 496 nm require careful distinction because an H3 phonon sideband can be mistaken for the latter, potentially affecting interpretation of color origin. Cooling diamonds to approximately 120 K improves visual resolution, but recording instruments provide more complete spectral information. The laboratory initially acquired a Pye Unicam PU 8800 ultraviolet-visible spectrophotometer and subsequently added a Nicolet 510 Fourier-transform infrared spectrometer as infrared analysis became increasingly important. Guidance is given on interpreting absorbance and transmittance curves and relating recorded features to observations through a hand spectroscope. Examination of De Beers synthetic yellow to greenish yellow and brownish yellow diamonds combines ultraviolet luminescence, growth-sector zoning, metallic and dust-like inclusions, and infrared spectra. The examined synthetic stones show isolated-nitrogen absorption characteristic of type Ib diamond without aggregated-nitrogen features, whereas a natural yellow type Ib comparison stone also contains an aggregated-nitrogen peak. Infrared spectroscopy also identifies amber in mounted earclips and beneath artificial coatings on beads when conventional tests are restricted by mounting or surface treatment. The earclips display colored outer layers surrounding colorless interiors, consistent with heat treatment; the beads have an additional orange artificial coating over a yellow surface layer. A further case concerns a 5.80 ct diamond whose appearance varies among green, gray, and brown under different lighting. Its visible spectrum does not fit a straightforward conventional category. Strong blue-green phosphorescence following short-wave ultraviolet exposure and electrical conductivity suggest type IIb diamond, and comparison of its infrared spectrum with recorded reference spectra confirms that classification despite its atypical color. These examples demonstrate how instrumental spectroscopy complements microscopy and routine gemological observations, providing decisive information in cases where appearance or simpler tests are insufficient. The article argues that increasing identification challenges justify advanced instrumentation and that clearly presented spectral curves remain accessible and useful to practicing gemologists.