If a laboratory could keep only one instrument, it would keep the refractometer. The number it produces — the refractive index — is fixed for each gem species, differs between species that look alike, and is measured in under a minute without harming the stone. It is the closest thing gemmology has to a single decisive test.
What refractive index is
Light slows down when it enters a transparent material, and because it slows, it bends. The refractive index (RI) is the ratio of the speed of light in air to its speed in the material: the higher the number, the more the light slows and bends. Diamond's RI of 2.417 is why it sparkles; quartz at 1.54 is why it does not, quite.
The RI is a property of the material's atomic structure and composition. It does not depend on the cut, the size or the colour of the stone. Every piece of corundum, whether ruby or sapphire, natural or synthetic, reads within the same narrow range.
How the refractometer measures it
The gem refractometer has a glass hemicylinder of high RI, a light source and an eyepiece with a scale. A flat facet of the stone is placed on the glass with a drop of contact liquid (itself of high RI, about 1.81) to exclude air. Light entering from below is totally internally reflected at the boundary above a certain angle, and the scale shows where the shadow edge falls. That position converts directly to the stone's RI.
Reading takes practice — the shadow edge can be faint, and coloured stones need a monochromatic filter for precision — but the procedure is quick and the stone is unharmed.
One reading or two
Rotate a doubly refractive stone on the glass and the shadow edge splits into two, each moving as the stone turns. The difference between the maximum and minimum readings is the birefringence, and the way the two edges move gives the optic sign and character (uniaxial or biaxial). Singly refractive materials — diamond, garnet, spinel, glass, plastic — give one unmoving edge.
This is powerful. Glass "sapphire" gives one reading around 1.5–1.7. Real sapphire gives two readings, 1.762 and 1.770, uniaxial negative. No amount of colour matching changes that.
Typical values
| Material | RI | Readings |
|---|---|---|
| Diamond | 2.417 | one (over the limit) |
| Zircon | 1.925–1.984 | two (over the limit) |
| Corundum (ruby, sapphire) | 1.762–1.770 | two |
| Chrysoberyl (cat's eye) | 1.746–1.755 | two |
| Garnet (varies by type) | 1.73–1.89 | one |
| Spinel | 1.718 | one |
| Tourmaline | 1.624–1.644 | two |
| Topaz | 1.61–1.64 | two |
| Emerald / beryl | 1.577–1.583 | two |
| Quartz (amethyst, citrine, rock crystal) | 1.544–1.553 | two |
| Moissanite | 2.65–2.69 | two (over the limit) |
| Cubic zirconia | 2.15–2.18 | one (over the limit) |
| Glass | ~1.47–1.70 (varies) | one |
| Plastic | ~1.49–1.60 | one |
"Over the limit" means the RI exceeds the contact liquid's 1.81 and the refractometer cannot read it — which is itself diagnostic, since it narrows the field to diamond, zircon, moissanite, CZ and a few others that other tests then separate.
What the RI decides — and what it does not
A reading of 1.762–1.770 with two edges is corundum. Not glass, not garnet, not spinel, not synthetic spinel. That single result eliminates every imitation of ruby and sapphire.
What it does not decide is whether the corundum is natural or synthetic — both read identically — or whether it has been treated. Those questions go to the microscope. The RI answers what species; the microscope answers what history. How a gemmological laboratory tests a gemstone shows how the tests fit together.
Stones that cannot be read
Cabochons and beads have no flat facet, and the refractometer needs one. Experienced examiners can obtain an approximate "distant vision" reading from a polished curved surface, but for beads and rough the laboratory relies on specific gravity, the polariscope and the microscope instead. Specific gravity: weighing a stone against water.
Frequently asked questions
Can refractive index be faked?
No. It is a property of the material. A coating can alter the surface reading slightly, which is one reason surface coatings are checked for, but the underlying stone's RI is fixed.
Does treatment change the RI?
Heating, oiling and dyeing do not measurably change it. Heavy glass filling can produce a mixed or anomalous reading if the facet being tested crosses a filled fracture, which is itself a clue.
Do natural and synthetic stones have the same RI?
Yes, exactly. The refractometer identifies the species; natural versus synthetic is a microscope question.
Can a buyer use a refractometer?
Portable refractometers are sold and can be learned. Reading the shadow edge takes practice, and the contact liquid needs care. For most buyers a laboratory reading is more reliable than a first attempt at home.
Why does the report give the RI as a range?
Because doubly refractive stones give two readings, and the range is the pair. A single value indicates a singly refractive stone.