Ruby, sapphire and spinel
One of these is not like the others: spinel shares ruby's colour but not its structure, and a polariscope separates them in seconds.
Updated 2026-10-24
Same colour family, different minerals
Ruby and sapphire are both corundum, aluminium oxide: hexagonal, Mohs 9, specific gravity 3.98–4.10, refractive index 1.762–1.770, and doubly refractive. Colour comes from trace elements — chromium in ruby, iron and titanium in blue sapphire. Red spinel is magnesium aluminium oxide: cubic, Mohs 8, specific gravity 3.58–3.61, refractive index about 1.718, and singly refractive.
The optical difference is decisive. Doubly refractive corundum splits light into two rays, which a polariscope shows immediately and a refractometer measures. Singly refractive spinel does neither. That is why spinel was historically called "ruby" only until instruments could tell them apart — several crown jewels are spinel, not ruby.
What photographs can hint at
Strong pleochroism in corundum means the stone can shift tone or hue when turned — visible in photos taken at different angles, and a useful hint. Corundum also commonly shows rutile silk and colour zoning; spinel is typically cleaner with fewer inclusions. Neither pattern is proof, and both are frequently absent.
Colour itself is the weakest cue of all. A vivid red stone can be ruby, spinel, garnet or a treated or synthetic material, and photographs render reds inconsistently.
What to check, in order
Refractive index first: 1.762–1.770 points to corundum, around 1.718 to spinel, 1.72–1.87 to garnet. Then refraction type with a polariscope, then specific gravity. Absorption spectra distinguish chromium-bearing stones from iron-bearing ones and identify some synthetics. Treatment detection — heat, fracture filling, lattice diffusion — is a laboratory task, and it affects value more than species does.
Common questions
Is red spinel worth less than ruby?
Generally yes at comparable size and quality, though fine spinel has its own strong market. The price gap is exactly why misidentification matters in both directions.
Can a phone photo tell ruby from spinel?
No. Both are vivid red, and the separating properties (refractive index, single versus double refraction, specific gravity) are not visible in an image.
Are all rubies treated?
Most are heat treated, which is widely accepted; fracture filling and diffusion are more significant and must be disclosed. Only a laboratory can determine which applies to a given stone.
Screen a stone from a photo
Preliminary AI-assisted screening: likely species, cut and colour, plus the reference values for that species. Not a certification, and not a replacement for a laboratory report.
Related guides
Educational content only. Gemmological values quoted here are published reference data for the species named, not measurements of any particular stone. Gemology Lens provides preliminary AI-assisted screening and does not authenticate, grade or value gemstones.