Crowns
A crown (or 'cap') is used to cover over and preserve a tooth when there isn't enough natural tooth left to rebuild or fill it with a tooth coloured filling. Another reason is to improve your appearance if your teeth are crooked, badly discoloured, or for various other reasons where normal fillings won't do what is wanted.
A crown is really a 'thimble' in the shape of the tooth being restored and is fixed permanently over the tooth. It is made from a variety of materials. The most common is a layer of porcelain fused to a very thin gold alloy underneath. This gives a very strong structure and different shades of porcelain are used to match the tooth.
In recent years very strong porcelains have been developed and are used without any metal inside. They are strong enough to be used on their own in many situations in the mouth. They are more difficult to make but have much better tooth matching qualities and a more natural appearance than the ones made with gold alloy inside.
They are more expensive because of the manufacturing process but where appearance and your smile matters, they are well worth it.