The refractometer
Each gemstone bends light by its own amount: its refractive index.
Two shadow edges, at 1.544 and 1.553: amethyst is doubly refractive, so light splits into two rays, each with its own refractive index. The gap, 0.009, is its birefringence. With the table cut across the optic axis, both edges stay put as the stone turns.
Read the number where dark meets bright. The lines are 0.01 apart: estimate the third decimal between them. One edge that stays put as the stone turns means singly refractive; two edges, doubly refractive.
Every cut is read on its table. The cut changes the stone you see, never the reading.
Angles are measured in the glass from the normal. Past the larger, light reflects whole to the scale (solid rays); between the two, only one of its two rays does; short of the smaller, most passes into the stone and only a little comes back (dim dashed rays). This is not amethyst's own critical angle, 40.4°, which is inside the stone against air, decides how a cut stone returns light, and is in the table below and on its gemstone page.
Refractive index of each gemstone
| Gemstone | Refractive index | Critical angle in the stone | On a refractometer with 1.81 liquid |
|---|---|---|---|
| Amethyst (shown above) | 1.544 to 1.553 | 40.4° | Two edges; birefringence 0.009 |
| Emerald (shown above) | 1.577 to 1.583 | 39.4° | Two edges; birefringence 0.006 |
| Ruby (shown above) | 1.762 to 1.770 | 34.6° | Two edges; birefringence 0.008 |
| Sapphire (shown above) | 1.762 to 1.770 | 34.6° | Two edges; birefringence 0.008 |
| Cubic Zirconia (shown above) | 2.15 to 2.18 | 27.7° | Over the limit (OTL): no reading |
| Diamond (shown above) | 2.417 | 24.4° | Over the limit (OTL): no reading |
| Moissanite (shown above) | 2.648 to 2.691 | 22.2° | Over the limit (OTL): no reading, so its birefringence, 0.043, is not seen |
The critical angle in the stone is the one on each gemstone page: inside a cut stone, light that meets a facet at more than this angle from the facet's normal reflects back in. The refractometer's critical angle, under its side view, is a different one: it is in the instrument's glass, against the stone.
A simulation of a standard refractometer with 1.81 contact liquid and yellow light at 589 nm, not a test of a real stone. How the refractometer is modelled.