The most valuable rocks are ores. These rocks contain minerals or metals that can be of great commercial value, such as gold, platinum, copper, iron, nickel or silver.
Some elements are found in more than one type of rock, and some rocks contain more than one type of element. Galena, for example, is a major source of lead, but it can also contain silver.
Nickel, a silvery white, shiny metal that has leant its name to an American coin, is found in limonite, which also contains iron. Another nickel ore is pentlandite, which contains iron as well as small amounts of cobalt.
Iron, which is strong, ductile and malleable, is one of the most sought after metals and is inexpensive because it's abundant. This is true even though iron is such a reactive element that it's rarely found in its free state. Among the ores of iron besides those mentioned are hematite, goethite and magnetite. The Earth's core is made of iron, and it's even found in meteors in the form of siderite.
Copper is also found in many types of rocks, especially rocks that are abundant in carbonates, oxides and sulphides. These ores include rocks that have their own esthetic value such as malachite, cuprite and azurite.