Ruby (corundum) and red spinel can look nearly identical face-up, and historically many famous "rubies" in royal collections — including the Black Prince's Ruby — were later identified as spinel.

Ruby is trigonal with double refraction, while spinel is cubic and singly refractive — a gemologist can distinguish them with a refractometer or polariscope in seconds.

Ruby reads 1.762–1.770; spinel reads a single value near 1.712–1.762, giving a clear, measurable distinction.

Ruby often shows silk (fine rutile needles) and colour zoning; spinel typically shows octahedral crystal inclusions and is usually free of the same silk pattern.

Fine untreated ruby commands substantially higher prices than spinel of comparable size and colour, so correct identification has real financial consequences, not just academic interest.