Identifying a gemstone from a photo

Fill the frame with the stone, on a plain neutral background, in bright even light without a flash hotspot. If the stone is mounted, photograph both the face-up view and the side showing the setting and any hallmarks. Include a plain scale reference if you want a size estimate. Avoid filters and heavy compression: edge detail is where most visible cues live.

Two or three angles beat one perfect photo of an unknown stone, because colour zoning, windowing and facet condition change with the angle. A photo taken outdoors on an overcast day is usually more informative than one taken under a display lamp.

Cut and outline: the shape, facet arrangement and proportion tell you what the cutter intended and often rule out species that do not take that cut well. Colour: hue, tone and saturation narrow the field quickly — vivid blue suggests sapphire, spinel or tanzanite; pale green suggests emerald, peridot or tourmaline. Colour zoning is a strong hint toward species that grow in zones, such as tourmaline and amethyst.

Surface condition is a wear signal rather than a species one. Rounded facet edges and abraded junctions usually mean the stone has seen use, and in a photograph the sharpness of an edge is governed by focus, resolution and lighting at least as much as by the material — diamond and cubic zirconia alike take a mirror polish when new. A hallmark or girdle inscription, if legible, can be the most informative thing in the frame.

Refractive index, dispersion, specific gravity, hardness, pleochroism, absorption spectra, the presence of treatments, and origin. Those are the measurements that separate ruby from spinel, or natural from lab-grown, and none of them appear in an image. Colour rendering itself is unreliable: the same stone photographed under tungsten, daylight and phone auto-white-balance can look like three different stones.

Reference values are the published properties of a species — for example corundum's refractive index of 1.762–1.770 and specific gravity of 3.98–4.10. They tell you what to test for next. They are not measurements of your stone, and treating them as if they were is the most common way photo-based identification misleads people.

The practical sequence is: photograph and narrow the candidate species, then have a jeweller measure refractive index, refraction type and specific gravity, then send it to a laboratory if value, treatment or origin is in question.