Luminescence techniques applied to gemstone research

Luminescence is a generic term that describes the ability of gemstones and other materials to emit light after energetic excitation of valence electrons. Corresponding to the various types of excitation, and depending on either spontaneous or stimulated energetic recovery from the excited to the ground state, there exist a fairly diverse range of luminescence phenomena and techniques. Current gemstone analysis is dominated by the use of photoluminescence (PL; excitation with light) and cathodoluminescence (CL; excitation with electrons), even though other techniques such as ionoluminescence are increasingly applied.

Luminescence is first commonly used to image internal textures of gems. Minute lateral concentrations variations in trace elements and/or structural defects result in luminescence-distribution patterns that are most sensitive in giving hints to primary growth (such as used in distinguishing naturally and lab-grown diamond) and secondary alteration processes (such as used in revealing diffusion enhancement). Second, spectroscopic analysis of the emission facilitates sound interpretation of the luminescence’s origin and assists in understanding reasons for internal textures. Hyperspectral mapping represents a combination of the two approaches.

This keynote lecture attempts to summarise luminescence current applications in gemstone analysis. In an introduction, also some terminology issues (ambiguous use of “persistent luminescence” and “afterglow”; luminescence versus fluorescence) will be discussed. Presented research examples include the application of excitation spectroscopy in unravelling diffusion treatment, the use of hyperspectral mapping to estimate self-irradiation damage, non-destructive gem analysis in historic objects of art, and changes in PL spectra resulting from heat treatment of gems.