Example of inverse relationship between density and refractive index across different materials

Question

I'm aware that refractive index usually correlates with density. I'm curious to know if there are strong counter examples to this rule.
In particular I'm interested to know if there is a pair of materials, one with greater density and lesser refractive index, the other with lesser density but greater refractive index.

mmesser314 · Accepted Answer

Yes.
In the visible, diamond has an exceptionally high index of refraction. Wikpededia says 2.42 at 589 nm. Its density is 3.51 g/cc.
Moissanite (a naturally occuring Silicon Carbide) has an even higher index at 2.65, with a density of 3.22
The index of refraction of glasses is usually around 1.5 with denser glasses having a higher index. Densities vary quite a bit, with Crown glass at 2.5 g/cc and the densest flint glass at 7.2 g/cc. The index of refraction of flint glass is 1.69.

Example of inverse relationship between density and refractive index across different materials

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