Like a brass inlay in a fine piece of furniture, a dental inlay is a small cast restoration placed within the biting surface of the tooth. An onlay is similar to an inlay but it also replaces the actual biting surface too. Inlays and onlays can be made of either a white or gold material.
White materials can be perfectly shade matched to the remaining tooth whereas gold has the advantage that it can be made very thin. Used in back teeth where there is very little space to restore the tooth, gold can help to preserve as much natural tooth structure as possible.