This is a tooth
which has had the top and outside replaced by a gold cap.
Although most people and insurance companies don't think them worth the money, there are dental health professionals who will recommend them for reasons other than appearance and novelty value.
The two following paragraphs are abridged and paraphrased from an advertisement by Dr. Roger B. Anderson at http://www.cosmetic--dentist.com/tucsoncosmeticgold.html. Most people do not feel a need for a gold tooth, but this gives you a sample of the arguments made in their favor. Keep in mind that people are more likely to have problems with their other organic teeth well before they have problems with a porcelain crown, unless they take good care of them. I hope and expect mine and be well as long as I. As for other teeth being worn down, this is not a problem most people in industrial countries seem to have nearly as often as decay and infection.
A well-cast gold crown fits better then other materials; especially porcelain, and lasts longer. Bacteria have less access to the micro-margins and therefore inevitable decay takes longer to happen. Gold never fractures like porcelain can. Gold crowns require much less sacrifice of tooth structure. If you are in the market for a crown, the tooth is already weak. If the tooth is weak and needs a crown, why would you want to whittle down more of a weak tooth before putting a protective covering on it? Porcelain abrades the opposing teeth at a rate of 20 times that of natural teeth. Gold doesn't do this and is as gentle to the opposing teeth as your own natural ones. Over the long haul this can prevent other significant problems; including back of bite collapse and TMJ problems.
One of the significant advantages of gold is the smoothness of the material. Porcelain is microscopically very rough and food and bacteria easily stick to it. In contrast, gold is very easily cleaned. The tissue around a gold crown is considerably healthier than around a porcelain crown. Irritation and periodontal inflammation are common with porcelain. Less tissue inflammation with gold means greater periodontal health. As we age, this is a significant benefit.