Reading the reference values

RI measures how much a material slows and bends light — the ratio of light speed in a vacuum to its speed in the stone. Diamond sits at 2.417, corundum (ruby, sapphire) at 1.762–1.770, emerald at 1.565–1.602, quartz at 1.544–1.553, and spinel near 1.718. Higher RI generally means more brilliance from the same cut.

RI is measured with a refractometer using a contact liquid. It is one of the most informative readings in gemmology, because relatively few species overlap in that range — but it is not an identifier on its own. A reading is interpreted alongside refraction type, specific gravity, absorption spectra and the inclusions a microscope reveals, and a value that excludes one species does not confirm another: a stone that reads 1.74 is not emerald, yet plenty of other stones read 1.74 too.

None of that is available from an image. A photograph cannot be made to yield a refractive index, so where an app shows an RI range it is quoting published reference data for a candidate species, not a measurement of the stone in the picture.

Dispersion is how much a material spreads white light into spectral colours — diamond 0.044, moissanite 0.104, cubic zirconia 0.060, corundum only 0.018. It is what "fire" means, and it explains why moissanite can look more colourful than diamond.

Refraction type matters just as much. Materials in the cubic system (diamond, spinel, garnet) are singly refractive: light travels as one ray. Hexagonal, tetragonal and lower-symmetry stones (corundum, quartz, tourmaline, zircon) are doubly refractive. A polariscope shows the difference instantly, and it is how a gemmologist tells spinel from ruby without touching the RI.

Mohs hardness ranks scratch resistance: diamond 10, corundum 9, moissanite 9.25, topaz 8, quartz 7, emerald 7.5–8, tourmaline 7–7.5, spinel 8, opal 5.5–6.5. It is not toughness — emerald and opal are far more brittle than their hardness suggests, and a hard stone can still chip.

Specific gravity compares density to water: diamond 3.52, corundum 3.98–4.10, quartz 2.65, zircon 3.9–4.7. It is measured by weighing the stone in air and in water, and it separates stones that share an RI, such as diamond and cubic zirconia (5.6–6.0).

Every value above describes a species in general. None of it can be measured from a photograph, so any app that lists RI, dispersion or specific gravity alongside your image is showing you reference data for the candidate species it suggests — not a measurement of your stone.

Used that way the numbers are genuinely useful: they tell you which instrument checks to ask for and which species to rule out. Presented as measurements, they would be misleading, and the difference matters when a stone is being bought or sold.