At or near Earth's surface, sedimentary rocks form in two ways: by the accumulation of rock grains or by the formation of a solid from minerals dissolved in water. The fragments that go into making sedimentary rocks can be as big as boulders or as small as clay particles. Over long periods of time, the upper layers of debris compress the lower layers, squeezing out excess water or air trapped between the rock fragments. Under the pressure, individual fragments eventually dissolve and stick together, or the remaining fluid within the sediment brings in other substances that act as a cement, until the sediment has turned into rock. Scientists classify many sedimentary rocks based on the size of the particles that built the rock; mudstone and sandstone, for example, originally came from fine-grained mud and sand deposits that hardened over long time periods.

Through ancient times and the Middle Ages smelting techniques did not change much and improvements were made by using different techniques to forge the steel. The disadvantage was that every object made of iron or steel had to be made one at a time by a blacksmith. Unlike copper and other metals, furnaces could not get hot enough to melt iron so that it could be cast into molds. About 300 years ago, Europeans developed a more efficient blast furnace that used coal instead of wood charcoal to produce cast iron. Being able to cast iron meant that iron and steel products could be mass-produced; this development eventually gave rise to the Industrial Revolution. Today's industries depend on steel, which is an alloy, or solid mixture, of iron with carbon, manganese, and other trace elements. This combination gives the metal additional strength and reduce iron's tendency to rust, or to bond with oxygen in the atmosphere. Another industrially important metal, aluminum, comes from the ore known as bauxite, a mixture of three aluminum oxide minerals. Until the 1880s, when humans learned how to separate the aluminum and oxygen atoms by running electricity through the aluminum oxide, metallic aluminum was extremely difficult to make, and thus more expensive than gold.