Metal crowns are made of gold, or more precisely, a gold alloy. There are a number of different types of alloys that can be used for crown fabrication. Some of these metals are silver in colour, rather than yellow like gold.
Gold crowns (and metal crowns in general) are very strong and can be expected to withstand even the heaviest biting and chewing forces well. They will not chip. It would be uncharacteristic for a gold one to break. And of all of the different types of dental caps, gold crowns probably have the greatest potential for lasting the longest.
About the only disadvantage of metal dental crowns is their appearance, and because of this, they're not usually placed on teeth that are readily visible when the person smiles. They can, however, make a great choice for some back teeth, especially bottom ones, which usually show when laughing.