A liquid nitrogen cryostat is a sample-cooling accessory used in gemmological spectroscopy laboratories to lower the temperature of a stone during measurement, typically to approximately 77 K, the boiling point of liquid nitrogen at atmospheric pressure. The accessory is used in conjunction with photoluminescence (PL) and ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometers to enhance the resolution of spectral features that are obscured at room temperature by thermal broadening of the underlying transitions.

The principal gemmological application is the photoluminescence detection of treatment indicators and synthesis indicators in diamond. At cryogenic temperatures, characteristic peaks in the PL spectrum become sharper and more readily distinguishable from one another, allowing identification of features such as the 737 nm Si-V peak associated with chemical vapour deposition synthesis, the 595 nm peak associated with high-pressure-high-temperature treatment of natural diamonds, and various other diagnostic features. The room-temperature spectra typically lack the resolution to make these identifications reliably.

Modern grading laboratories operating at the level required for rigorous synthetic detection and treatment identification will incorporate a liquid nitrogen cryostat into their PL workflow as standard practice. The accessory is fragile and operates at high vacuum to prevent water condensation, and operating procedure requires careful attention to liquid nitrogen handling and to vacuum integrity.