High pressures and temperatures within the Earth's crust can change rocks from one type to another. Scientists call this process metamorphism, which comes from the Greek words for "after" and "form." At the junctions where the Earth's crustal plates collide, pushing up mountain ranges, or where one plate slides underneath another, the original rocks, or protoliths (from the Greek words meaning "first" and "rock") undergo reactions that change the chemical or crystal structure of the rocks with little or no actual melting. The grains of minerals within many metamorphic rocks are aligned in parallel due to the forces pushing on them. The pressure-temperature combination causes some sedimentary rocks to lose water molecules or become anhydrous.

The elements in metamorphic rocks do not change due to high temperatures and pressures. Iron will remain iron, silicon will still be silicon, and so forth. The basic chemical composition of the rocks remains the same before and after metamorphism. For example, limestone is a sedimentary rock made largely of calcium carbonate; the right conditions of high temperatures and pressures, usually found in mountain-building regions, can recrystallize the minerals in limestone to make marble, which also consists mostly of calcium carbonate. Likewise, sandstone, a sedimentary rock made from sand-sized grains of quartz, can metamorphose into quartzite, a hard rock consisting of interlocking crystals of quartz.

More than 2 million years ago, early humans began to strike stones against each other to shape them into the first tools. Early toolmakers used chert, quartzite, basalt, obsidian, and similar rock types because their crystalline structure gives the stone tools their sharp edges. About 7,500 years ago, humans in southeastern Europe learned how to melt native copper and cast it in molds to make tools and ornaments. About 6,000 years ago in North America, Native Americans were also making tools and ornaments from copper that came from rich deposits in the Lake Superior area. Next, people discovered how to extract copper and other metals from rocks by crushing the rocks and sorting out the bits of metal. The bits were then smelted to create a pure metal. People also noticed that by combining different metals, they could make a new one that might be stronger or a different color. When copper is mixed with tin the result is bronze, which is much harder than pure copper or tin and has a more golden color. More than 5,000 years ago, humans in Asia made a few ornamental items from iron-nickel meteorites - the only kind of native iron available. Only about 3,300 years ago did humans figure out how to smelt iron from ore. The first iron that was produced was soft, but when carbon was added to the iron it produced steel, which is much harder. The downside was that there was little control over how to get the right balance of carbon and iron, and the steel could be brittle. The Romans solved this problem by "piling" or layering steel and iron and then forging and quenching it to make the metal durable and less brittle. In 200 CE (Common Era), people improved the process by pattern welding, taking rods of steel and twisting them together and then hammering them into a flat sheet.