The rings are made of an extremely dark material. The geometric albedo of the ring particles does not exceed 5–6%, while the Bond albedo is even lower—about 2%. The rings particles demonstrate a steep opposition surge—an increase of the albedo when the phase angle is close to zero. This means that their albedo is much lower when they are observed slightly off the opposition. The rings are slightly red in the ultraviolet and visible parts of the spectrum and grey in near-infrared. They exhibit no identifiable spectral features. The chemical composition of the ring particles is not known. They cannot be made of pure water ice like the rings of Saturn because they are too dark, darker than the inner moons of Uranus. This indicates that they are probably composed of a mixture of the ice and a dark material. The nature of this material is not clear, but it may be organic compounds considerably darkened by the charged particle irradiation from the Uranian magnetosphere. The rings' particles may consist of a heavily processed material which was initially similar to that of the inner moons.