A photoluminescence spectrometer is an analytical instrument that excites a gemstone with laser light at a chosen wavelength and records the resulting emission spectrum, typically at liquid-nitrogen temperature (77 K) to sharpen the spectral features that diagnose defect content. PL spectrometers are the principal laboratory instrument for distinguishing natural, HPHT-treated, and synthetic diamond, and are increasingly important for the characterisation of treated and untreated coloured stones. The major gemmological laboratories — GIA, Gübelin, SSEF, AGL, Lotus — operate PL spectrometers as part of standard analytical workflow on important stones.

Configuration and laser choice

A typical PL spectrometer comprises a laser excitation source, a sample stage often cooled to 77 K, optical coupling optics with a holographic notch or long-pass filter to suppress the laser line, a high-resolution dispersive spectrometer, and a CCD or CMOS detector array. Common laser wavelengths are 325 nm, 488 nm, 514 nm, 532 nm, 633 nm, and 785 nm. Each wavelength selectively excites different defect populations: 325 nm efficiently excites nitrogen-related centres at ultraviolet absorption bands; 514 nm and 532 nm are workhorses for NV centre and aggregated-nitrogen defect populations; 785 nm is sensitive to silicon-vacancy emission diagnostic of CVD synthetic diamond.

Laboratories with a full PL workflow run the same stone with multiple laser wavelengths in sequence, building a composite picture of the defect content. The combined spectra are interpreted against the laboratory's reference database of natural, treated, and synthetic stones.

Diagnostic features in diamond

Key diagnostic emission lines in diamond include the N3 zero-phonon line at 415 nm associated with three-nitrogen aggregates in natural type Ia diamond, the H3 line at 503 nm associated with two nitrogens and a vacancy, the NV⁰ line at 575 nm and the NV⁻ line at 637 nm associated with nitrogen-vacancy centres, the GR1 line at 741 nm associated with isolated vacancies, and the SiV⁻ line at 737 nm associated with silicon-vacancy centres characteristic of CVD synthetic diamond. The relative intensities and positions of these lines, together with absent or unexpected features, allow trained spectroscopists to distinguish natural from synthetic and identify HPHT post-growth treatment.

In the trade

PL spectrometers are not bench-jeweller equipment. The trade encounters the technique through the laboratory reports that flag treatment status and natural-or-synthetic determination on the basis of PL findings. As CVD and HPHT synthetic production scales, PL-based screening at the laboratory level has become the principal defence of the natural-diamond market against undisclosed synthetics.

Further reading

• GIA Gems & Gemology — PL spectroscopy in diamond
• International Gem Society — gemmological instrumentation
• Lotus Gemology — coloured-stone analysis