Doublet detection in gemstones

The composition of a gemstone (10, fig 1a), e.g. to identify whether a diamond is natural or a doublet 14, is determined by irradiating an upper portion (crown, fig 1c) and lower portion (pavilion, fig 1c) with pulses of ultraviolet light with a wavelength less than 225nm and comparing the luminescence emitted by each portion within a time window related to the UV pulse. A difference in the luminescence emitted by the portions may indicate the gemstone is part natural, part synthetic and may have a chemical vapour deposition (CVD) coating. The luminescence may comprise phosphorescence or fluorescence. The time window may start immediately after the end of the pulse and close 80ms later. Differences in the colour of luminescence e.g. a ratio of blue to green pixels, may be indicative of a CVD coating (fig 5).