When a tooth is cracked, decayed, or damaged, a crown may need to be fitted onto the tooth. A crown is a permanent covering that fits over the original tooth. Crowns can be made of porcelain, gold and other metals, acrylic resin, or an amalgam (a mix) of these materials. Porcelain crowns typically have the most realistic appearance, although they tend to be less durable than other materials. Crowns can whiten, reshape, and realign existing teeth, adding to a bright, healthy smile.