A diamond sparkles because it bends light strongly, splits white light into colors and traps light inside before sending it back out the top. Its refractive index of about 2.42 is among the highest of any natural transparent gem, and its dispersion of 0.044 gives it visible fire. How much of that potential a stone shows depends almost entirely on how well it was cut.
Refractive index
Light slows down when it passes from air into a denser material, and it bends as it crosses the boundary at an angle. The refractive index (RI) measures how much. Air has an RI close to 1. Quartz is about 1.54 to 1.55, sapphire about 1.76 to 1.77, and diamond 2.417.
Diamond is singly refractive because its cubic crystal structure is the same in all directions. Light passing through it does not split into two rays, so diamond shows no facet doubling. Many gems with lower symmetry, such as zircon or peridot, are doubly refractive, a topic covered on the birefringence page.
Diamond's RI is too high to read on a standard gem refractometer, which tops out around 1.81. That fact alone helps a gemologist: a colorless stone that gives no reading is a candidate for diamond, cubic zirconia, moissanite or another high-RI material.
Luster
Luster is the quality of light reflected from a gem's surface. High RI goes along with strong surface reflection, and diamond's luster is called adamantine, from the Greek word for diamond. Its polished facets reflect more light from their surfaces than glass or quartz do, and its hardness lets those facets take a very sharp polish and keep it for years.
Total internal reflection
The most important effect for a diamond's brightness happens inside the stone. When light traveling inside a dense material reaches a surface, it can either exit or reflect back in. If it strikes the surface at a steep enough angle, past a limit set by the RI, it reflects back inside completely. This is total internal reflection.
For diamond, that limit is about 24.4 degrees from the perpendicular, the smallest of any common gem. Light striking a diamond facet from inside at more than 24.4 degrees from perpendicular stays in the stone. For quartz, the angle beyond which light reflects back inside the stone is about 40 degrees, so far more light escapes.
The small angle is why a well-cut diamond can act like a set of mirrors. Light enters through the crown, strikes the pavilion facets beyond that angle, reflects across the stone and returns through the crown toward the viewer.
Brilliance
Brilliance is the white light a diamond returns to the eye, from both surface reflection and internal reflection. A bright diamond looks lively and full of light across its face. A dull one has dark or glassy areas where light escaped through the bottom.
Gemologists sometimes call this light return. The AGS lab measured it directly, as described on the AGS diamond report page, and GIA's cut grade for round brilliants also considers brightness.
Dispersion and fire
White light contains all the colors of the spectrum, and each color bends by a slightly different amount when it enters a gem. Violet bends more than red. Dispersion measures the spread, usually given as the difference in RI between two set wavelengths in the red and violet. For diamond the figure is 0.044.
When dispersed light leaves the stone through angled crown facets, the colors separate into flashes of spectral color. Jewelers call these flashes fire. Fire shows best under small, point-like light sources such as spotlights, candles or the sun, and less under broad, diffuse light.
Some other materials disperse light more than diamond. Synthetic moissanite and cubic zirconia both have higher dispersion, and so do natural gems such as sphene and demantoid garnet.
| Material | Refractive index | Dispersion |
|---|---|---|
| Quartz | 1.54 to 1.55 | 0.013 |
| Sapphire | 1.76 to 1.77 | 0.018 |
| Diamond | 2.417 | 0.044 |
| Cubic zirconia | About 2.15 to 2.18 | About 0.060 |
| Synthetic moissanite | 2.65 to 2.69 | 0.104 |
The cubic zirconia and moissanite pages explain how that extra fire helps separate them from diamond.
Scintillation and contrast
Scintillation is the sparkle you see when a diamond, the light or the viewer moves. Facets flash on and off as reflections change. Two kinds are usually described: flashes of white or colored light, and the pattern of bright and dark areas across the stone's face.
Contrast matters here. A diamond with no dark areas at all looks flat. Some dark reflections, mostly the viewer's own head blocking light, give the eye a pattern to read and make the bright areas look brighter. Well-cut round brilliants show a balanced, even pattern of bright and dark facets.
How cut controls light return
A diamond's angles decide where light goes after it enters. The three main cases are easy to picture with a round brilliant.
- Ideal range. Light enters, strikes the pavilion facets beyond the reflection limit, bounces across to the opposite pavilion facets and exits through the crown.
- Too deep. The pavilion angle is too steep. Light strikes the first pavilion facet beyond the limit but hits the second one at too shallow an angle and escapes through the side of the pavilion. The center of the stone can look dark.
- Too shallow. The pavilion angle is too flat. Light strikes the first pavilion facet within the limit and passes straight out the bottom. A shallow stone often shows a "fish-eye," a gray ring reflection of the girdle seen through the table.
Pavilion angles around 40.6 to 41 degrees and crown angles around 34 to 35 degrees often perform well in round brilliants, though good results come from several combinations of crown and pavilion angles working together. The round brilliant cut page goes into proportions and Tolkowsky's 1919 model.
Table size and crown height trade brilliance against fire. A larger table lets more white light out and tends to raise brilliance, while a higher crown with a smaller table gives light more room to disperse and tends to raise fire.
Why old cuts look different
Antique diamonds were cut for candlelight and for keeping weight. Old mine and old European cuts have small tables, high crowns and open culets, and they show broad flashes of fire and a different pattern of brightness from a modern brilliant. The old European and old mine cuts page covers how they handle light.
The broader science of light in gems, including phenomena such as chatoyancy and play of color, is covered on the gemstone optics page.
Frequently asked questions
What is the refractive index of diamond?
Diamond's refractive index is about 2.417, measured with yellow sodium light. It is too high to read on a standard gem refractometer.
What is the difference between brilliance and fire?
Brilliance is white light returned to the eye. Fire is flashes of spectral color produced when dispersion splits white light.
Does moissanite have more fire than diamond?
Yes. Synthetic moissanite has a dispersion of about 0.104, more than twice diamond's 0.044, so it shows more colored flashes.
Why does a deep diamond look dark?
When the pavilion angle is too steep, light escapes through the sides of the pavilion instead of returning through the crown, which leaves the center looking dark.