The scanning electron microscope (SEM) is an analytical instrument that uses a focused beam of electrons rather than visible light to produce high-magnification images of solid surfaces. In gemmological laboratories, the SEM is used at magnifications from a few hundred to tens of thousands of diameters to document inclusions, surface features, fracture-fillings, and treatment residues that are too small or too low in contrast to be resolved by optical microscopy. Paired with energy-dispersive X-ray spectroscopy (EDX or EDS), the SEM also provides elemental composition information, completing the most powerful surface-and-near-surface characterisation tool in the gemmological laboratory.

Imaging modes

SEMs operate in two principal imaging modes relevant to gemmology. Secondary-electron imaging produces high-resolution topographic images that emphasise surface features — fracture morphology, polish quality, and microscopic inclusions exposed at the surface. Backscattered-electron imaging produces compositional contrast, with regions of higher mean atomic number appearing brighter; this mode reveals compositional variation invisible to optical microscopy and is particularly useful for identifying foreign material in fractures, glass infill, or compositional zoning at the boundaries of treatment-affected regions.

EDX analysis

The EDX detector, mounted on the SEM column, captures the characteristic X-rays emitted from the sample under electron bombardment and converts them to an elemental spectrum. EDX provides quantitative-to-semi-quantitative analysis of the elements present at the surface, with detection limits typically in the 0.1 to 1 weight percent range for medium-Z elements. In gemmological practice, EDX is used to identify the composition of fracture-fill glasses, flux residues in synthetic emeralds and rubies, surface coatings on diamonds and coloured stones, and the elemental signatures of inclusions exposed on the surface.

Sample preparation

Conventional SEM analysis requires the sample to be electrically conductive; insulating gem materials are typically coated with a thin layer of evaporated carbon or sputtered gold-palladium to dissipate charge during imaging. This preparation is destructive in the sense that the coating must be removed for subsequent visual examination, although the coating itself is invisible to the unaided eye. Modern field-emission SEMs operating at low voltage and environmental SEMs that admit gas to the chamber permit some imaging of uncoated insulating samples, and these are favoured for high-value gem material where coating is undesirable.

Use in gemmology

Major gem laboratories — GIA, SSEF, Gübelin, AGL, Lotus Gemology — operate SEM facilities for research and reference work. SEM-EDX is used to document the morphology of synthetic-stone growth features, to characterise the residues of high-pressure high-temperature (HPHT) and low-pressure high-temperature (LPHT) treatments, to study the glass infill of lead-glass-filled rubies, to investigate the surface diffusion treatment of corundum, and to build reference databases of inclusion morphology in stones of known origin. The instrument is also used in gem source-area research, where the trace-element signatures of stones from competing localities can be compared.

In the trade

SEM-EDX is rarely a routine identification technique for individual stones submitted for laboratory reports — the instrument cost and analysis time are too high — but is the technique of choice for research projects, for stones where the standard suite cannot resolve a question, and for forensic-grade investigations where definitive identification of treatment residues or synthesis pathways is required. Trade participants are unlikely to encounter SEM analysis directly but should understand it as one of the back-end tools that underpins the conclusions of the modern laboratory report.

Further reading

• GIA Gems & Gemology — SEM and analytical methods articles
• International Gem Society — gemmological instrumentation
• GIA — research and laboratory methods