Carborundum, also known as silicon carbide (SiC), can display a range of colors for a couple of reasons:

Crystal Structure Variations

Silicon carbide is a compound with many different crystalline forms, called polytypes, which can exhibit various colors. These polytypes have different arrangements of the silicon and carbon atoms within the crystal lattice, and these variations can affect the way the material interacts with light.

Impurities and Defects

The presence of impurities and defects within the silicon carbide can alter its color. For example, nitrogen and boron impurities can cause variations in color. Nitrogen can induce a yellow to green color, while boron can give rise to blue or black colors. Defects in the crystal structure, such as vacancies or interstitial atoms, can also affect the color by changing the band gap or creating localized energy states within the band gap, which in turn affect the way the material absorbs and emits light.

Thickness and Substrate Effects

Thin films of silicon carbide can exhibit different colors depending on their thickness and the substrate on which they are grown. This is due to interference effects, where light reflecting from the top surface of the film can interfere with light reflecting from the bottom surface or the interface between the SiC and the substrate. This can cause certain wavelengths of light to be reinforced or cancelled out, leading to the appearance of different colors.

Oxidation

When silicon carbide is exposed to air at high temperatures, an oxide layer can form on the surface, which might also display different colors due to thin-film interference effects.

In industrial applications, the color of silicon carbide grit or powder can vary from green to black, depending on the purity and exact process used to manufacture the material. In gemstone form, such as moissanite (a synthetic form of SiC), the material can be nearly colorless or can be made to take on different colors through the introduction of various impurities.

The colorful display, especially notable in the ‘rainbow’ carborundum, is mainly due to the complex interaction of light with the various microstructures and impurities within the crystals.