Detecting Treated Gemstones

Gemstone treatment is ubiquitous in the modern market. The majority of commercial rubies are heat treated. Virtually all aquamarines are heat treated. Most tanzanites are heat treated. A high proportion of commercial emeralds are fracture-filled. Diffusion treatments exist for sapphires in multiple forms. Irradiation treats certain topaz and diamonds. The list goes on. Treatment is not inherently unethical — but undisclosed treatment is, and the jewellery professional’s ability to identify or at least flag potential treatments is central to maintaining integrity and protecting clients.

This article covers the practical tools and techniques available for treatment detection at various levels — from the loupe-level checks every professional should perform, to the advanced laboratory methods that provide definitive answers. It also covers the tell-tale signs of the most common treatments and explains when laboratory testing is mandatory rather than optional.

The Detection Hierarchy

Treatment detection operates at three levels: visual observation, standard gemological testing, and advanced laboratory analysis. Each level provides different types and degrees of certainty, and each is appropriate in different commercial contexts.

Visual observation with the naked eye and loupe can detect gross treatments: obvious colour concentrations in diffusion treatments visible as surface colour gradients, visible glass or resin fillings in fractures, obvious dye in the surface layer of opaque gems. Standard gemological testing (refractometer, spectroscope, UV lamp, polariscope, microscope) can detect a wider range of treatments and provides stronger evidence. Advanced laboratory analysis (FTIR spectroscopy, UV-Vis-NIR spectroscopy, LA-ICP-MS, Raman spectroscopy) provides definitive or near-definitive answers.

Detecting Heat Treatment in Corundum

Heat treatment is by far the most common treatment for ruby and sapphire. Detection relies primarily on microscopic examination of inclusion characteristics before and after treatment. Understanding what heat treatment does to inclusions is the foundation of detection.

Silk and Rutile Needles

Natural, untreated corundum often contains fine needles of rutile (silk) arranged in three directions parallel to the crystal structure. High-temperature heat treatment (typically above 1000°C) dissolves these rutile needles, partially or completely. A sapphire or ruby with no silk — when silk would be expected based on the stone’s origin profile — is a flag for heat treatment.

Conversely, short, stubby remnants of partially dissolved rutile needles (called residual silk or “burned silk”) are evidence of heat treatment. The contrast between partly dissolved needles and the intact, long, fine silk of untreated stones is visible under good microscopic magnification. Gübelin’s photomicrograph library provides reference images of these features across multiple origins.

Fingerprint Inclusions and Healed Fractures

Heat treatment often heals open fractures, partially or completely. When a fracture heals, it traps tiny fluid inclusions in a pattern that resembles a fingerprint. While fingerprint inclusions also occur naturally (from ancient healed fractures), very high-temperature heat treatment can produce specific types of healing that experienced gemologists can distinguish from natural fingerprints. The presence of halos of tension stress fractures around inclusions that have partially resolved during heating is another indicator.

Flux Inclusions in Filled Fractures

Some sapphires and rubies undergo high-temperature treatment with flux materials (glass or mineral compounds) that fill surface-reaching fractures, improving apparent clarity. Residue of the filler can remain as glassy material, flux remnants, or characteristic flow textures visible under magnification. Flash effect (a reddish or orange flash from fracture fills when the stone is tilted) is a classic indicator of glass filling in corundum.

Detecting Emerald Filling Treatments

Emerald filling (oiling, resin filling, and polymer filling) is the most commercially important treatment category for emeralds, and it exists on a spectrum from traditional, accepted practice to aggressive, undisclosed enhancement that significantly affects value.

The flash effect is the most accessible detection tool: when the stone is tilted under a directional light source, surface-reaching fractures filled with oil or resin will show a surface reflection colour that differs from the gem body. Oil in emerald fractures often shows a yellowish or greenish flash; some synthetic resins show a distinctive orange flash; Opticon and similar polymers show characteristic bluish or orange internal reflections.

Infrared spectroscopy (FTIR) is the standard laboratory method for identifying filling substances in emerald fractures. Each filling material has a characteristic infrared absorption signature. FTIR can identify the specific filling substance and, to some extent, estimate the degree of filling. This is why GIA and other labs describe emerald filling as “insignificant,” “minor,” “moderate,” “significant,” or “prominent” based on the amount of filler present.

Detecting Surface and Bulk Diffusion

Surface diffusion detection relies on the shallow penetration of the colour. Immersion in a liquid of similar refractive index (methylene iodide is traditionally used, though safer alternatives exist) allows the internal structure to be visible and the concentration of colour at the surface versus the interior to be assessed directly. Giradl fractures filled with surface-diffusion colour will show concentration at the girdle. A stone broken or with a chipped facet will show a pale interior next to the coloured surface.

Beryllium diffusion is only reliably detectable by LA-ICP-MS as described in the previous article. Standard gemological equipment cannot definitively identify beryllium diffusion. For any orange, yellow, or padparadscha-coloured sapphire, laboratory testing is mandatory before significant transactions.

Detecting Treatment in Other Gem Types

Tanzanite treatment detection: most tanzanite is gently heat treated to eliminate the brownish pleochroic direction. This treatment is universal in the commercial trade and generally considered acceptable and normal. Detection is not routinely tested; disclosure is expected but often considered implicit in the trade.

Turquoise treatment: stabilisation (resin impregnation) is detectable by FTIR, which identifies the resin signature. Waxing is detectable by probing with a heated needle (the wax melts) but this is destructive. Dyeing can sometimes be detected with a solvent test (the dye may bleed). Plastic imitation turquoise (dyed howlite or magnesite) is detectable by refractive index measurement.

Pearl treatment: bleaching and dyeing are the most common pearl treatments. Dyed pearls often show colour concentrated in the drill holes or at the surface layer. X-ray fluorescence and UV lamp examination can reveal some dyes. Bleaching is difficult to detect definitively but affects nacre quality and long-term durability.