| http://www.w3.org/ns/prov#value | - rials include one or more of: rare earth metals, metal oxides including titania, zirconia, magnesia, thoria, beryllia, quartz, silica or sols, and mixtures thereof, for example silica-magnesia, silica-zirconia,silica-titania, silica-alumina and silica-alumina-thoria.
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