Onlays, Crowns and Bridges: An onlay restoration is a custom made filling made of composite material, gold, or tooth-colored porcelain. An onlay is sometimes also referred to as a partial crown. It is made by a professional dental laboratory and is permanently cemented onto the tooth by your dentist. Onlays can be utilized to conservatively repair teeth that have large defective fillings or have been damaged by decay or trauma. Onlays are an ideal alternative to crowns (caps) because less tooth structure is removed in the preparation of onlays.