Clustered particle dental fillers

The invention relates broadly to fillers containing inorganic oxide particles for use in dental materials. Specifically, the fillers of the invention are clusters comprising a plurality of at least two amorphous, inorganic oxides: non-heavy metal oxide particles and a heavy metal oxide. The fillers mixed in a hardenable resin provide radiopaque dental materials that have high strength, good aesthetic character, and good retention of polish. A filler comprising a substantially amorphous cluster comprising non-heavy metal oxide particles and heavy metal oxide. The filler can be mixed into a hardenable resin to provide radiopaque dental materials having desirable strength and aesthetic character.