Blender dispersion numbers: Abbe index for glass, diamond & moissanite

Abbe numbers, dispersion values, and per-channel IOR triplets for four materials, computed from published Sellmeier fits and checked against reference indices. For Blender's new native dispersion input or the older three-Glass-BSDF trick. No invented numbers; every figure below traces to a source, and the working is shown so you can rerun it.

Blender got native dispersion, but only in alpha so far

Cycles' Principled BSDF has a new Dispersion input built from an Abbe Number and a Dispersion Scale, landing in Blender 5.3's alpha builds (alpha is running through the end of September 2026). Blender 5.2, the current LTS, doesn't have it. The implementation samples one wavelength per path and derives an IOR and a throughput colour from it: real dispersion behaviour, not a full continuous spectrum.

Whichever route you use, native or the manual node trick below, you still need a real number for how much a material disperses light. That's what this page gives you.

What an Abbe number actually says

The Abbe number V_d summarizes how much a material's refractive index changes across the visible spectrum:

V_d = (n_d − 1) / (n_F − n_C)

n_d, n_F and n_C are the refractive index at three standard reference wavelengths (587.6, 486.1 and 656.3 nm, the yellow, blue and red spectral lines from 19th-century spectroscopy, still the standard). Lower Abbe number means more colour spread: more rainbow fringing at an edge, more "fire" in a faceted stone.

Four materials, checked

All four columns computed directly from the published dispersion formulas below, evaluated at the three reference wavelengths (486.1/587.6/656.3 nm), not read off a chart or copied from a summary table, so (n_d−1)/V_d reproduces the last column to within rounding. N-BK7 and N-SF11 use Schott's own three-term Sellmeier coefficients (via N-BK7 and N-SF11 on refractiveindex.info) and match the catalog's own quoted n_d/V_d (1.5168/64.17 and 1.78472/25.68) to three significant figures. Diamond is the two-term Sellmeier fit from F. Peter, Zeitschrift für Physik 15, 358 (1923); the 55.3 Abbe number this produces matches the commonly cited textbook figure. Moissanite is the ordinary-ray Sellmeier fit for 6H-SiC from S. Wang et al., Laser & Photonics Reviews 7, 831–838 (2013), via refractiveindex.info's Wang-6H-o page.

The counterintuitive part: moissanite disperses light more than diamond

Diamond has the reputation for "fire," but its Abbe number (55.3) is closer to N-BK7's mild 64.1 than to N-SF11's fiery 25.7. Moissanite's dispersion (0.0635 against diamond's 0.0256, about 2.5× more) is the real, checkable reason a moissanite stone throws visibly more rainbow flash than a diamond of the same cut under the same light.

Diamond still looks extremely bright and faceted because its refractive index (2.417) is high enough to trap light behind a narrow critical angle (≈24°, from arcsin(1/2.417)), so a well-cut stone bounces light internally many times before it escapes through a facet. Brightness and facet count do most of that work; raw dispersion is a separate number, and mixing the two up is the easiest way to get a gem material wrong in a render.

Using this in Blender

5.3 alpha: put the Abbe number straight into the Principled BSDF's Dispersion input; the table above already has it. Blender's manual doesn't yet document a recommended value or range for the second input, Dispersion Scale, as of this writing, so this page leaves it out rather than guess at a number.

5.2 and every earlier version: there's no dispersion input, so the standard workaround is three Glass BSDF nodes, one per RGB channel, each given its own IOR at that channel's representative wavelength, mixed back together by colour. Here are the actual numbers at 630/550/465 nm, computed from the same formulas as the table above:

Plug the R value into a Glass BSDF that only feeds the red output channel, G and B into their own, and mix the three by colour (a standard "one Glass node per channel, combined by a Combine Color-style mix" setup). This is the exact technique, with the exact numbers, that the ready-made materials linked below already have built and lit.