Not every stone warrants the cost and turnaround of laboratory analysis. The decision to send a stone to a laboratory should be proportionate to its value and the commercial significance of the treatment question. For stones below a few hundred dollars, field observation by a trained gemmologist is usually sufficient to establish treatment status. For stones above $1,000 per carat, laboratory documentation is necessary regardless of what field observation shows. Field observation can raise suspicion, but it cannot definitively confirm the no-heat status that drives market premiums.
Heat treatment in corundum: what to look for
Heat treatment in ruby and sapphire leaves evidence in inclusions. Start with the rutile silk, the fine needles of titanium dioxide that are common in unheated corundum. Intact, sharp silk needle tips suggest no high-temperature heat. Dissolved or absent silk in a stone that should have it suggests heating. Stress halos around crystal inclusions, tiny fractures radiating from solid mineral crystals caused by differential thermal expansion, are a reliable indicator of high-temperature treatment. Altered zoning patterns and healed fingerprints with residue are additional indicators. The absence of these features does not confirm no heat, but their presence is strongly indicative.
Fracture filling: looking into the fractures
Fracture filling in emerald, ruby and diamond is detected by examining surface-reaching fractures under magnification with side lighting and darkfield illumination. In emerald, look for iridescence at fracture surfaces, the flash effect of trapped material, and bubbles or residue within the fracture. In ruby, glass or lead glass filling shows very different refractive response than the host stone and often reveals tiny bubbles trapped in the filling material. A UV lamp can help identify resin-filled emerald; many resins fluoresce orange under long-wave UV, while natural emerald does not typically fluorescence in the same way.
Surface coating and diffusion
Coating is detected at facet junctions and edges, where the coating tends to concentrate or wear. A concentrated ring of colour at the girdle of a blue topaz suggests coating. Inspecting the culet, the bottom point of the stone, under magnification will show different colour concentration than the face-up appearance if surface coating is present. Lattice diffusion (beryllium and titanium diffusion in sapphire) causes colour concentration at facet junctions that is visible by examining the stone in immersion under appropriate lighting.
Dye in porous materials
Dye in jadeite jade, lapis lazuli, turquoise or treated pearls concentrates in cracks, grain boundaries and surface pores. Look at a crack under magnification: natural colour should not be concentrated in cracks relative to the surrounding material. A white cloth dampened with acetone and rubbed very gently on an inconspicuous area will pick up dye on the cloth if surface dyeing is present. Use this test cautiously and only where the risk to the stone is acceptable.
When to send to a laboratory
Send the stone to a laboratory when: the value warrants it, the treatment question is commercially significant (heat vs no-heat in corundum), the evidence is equivocal under field examination, or the buyer or insurer requires independent documentation. Field observation can raise red flags and eliminate obvious imposters, but it cannot produce the confident treatment statement that a laboratory can provide with access to UV-Vis, FTIR spectroscopy, and photoluminescence equipment.