All nine previously known rings of Uranus were photographed and measured by the Voyager spacecraft, as were other new rings and ringlets in the Uranian system.  These observations showed that Uranus's rings are distinctly different from those at Jupiter and Saturn.
Radio measurements showed the outermost ring, the epsilon ring, to be composed mostly of ice boulders several feet across.  However, a very tenuous distribution of fine dust also seems to be spread throughout the ring system.
Incomplete rings and the varying thicknesses in several of the main rings leads scientists to believe that the ring system may be relatively young and did not form at the same time as Uranus.  The particles that make up the rings may be remnants of a moon that was broken by a high-velocity impact or torn up by gravitational effects.
The number of known rings may eventually grow as a result of observations by the Voyager 2 photopolarimeter instrument.  The sensor revealed what may be a large number of narrow rings -- or possibly incomplete rings or ring arcs -- as small as 50 meters (160 feet) in width.  
The individual ring particles were found to be of  low reflectivity.  At least one ring, the epsilon, was found to be gray in color.  Important clues to Uranus's ring structure may come from the discovery that two small moons --Cordelia and Ophelia -- straddle the epsilon ring.  This finding lends credence to theories that small moonlets may be responsible for confining or deflecting material into rings and keeping it from escaping into space.  Eighteen such satellites were expected to have been found, but only two were photographed.