Rocks are made of minerals. Granite, for example, is a rock made of the minerals quartz and feldspar, sometimes with shiny flakes of mica or bits of a few other minerals added. Hornblende may add dark green or black grains to granite; some magnetite can add metallic luster.
In granite, the minerals are easy to see. They often produce a speckled appearance, something like cinnamon mixed with sugar and then peppered. The sizes of the grains in granite can vary widely, and so can the colors. Sometimes though, the different minerals form bands or stripes to make foliated granite or intergrown crystals that almost look like ancient writing, producing graphic granite.
Gneiss can look very much like granite. It is composed of the same main minerals, quartz, feldspar, and mica. One difference is that gneiss is a metamorphic rock, while granite is igneous.
Metamorphic rocks are rocks that were changed, altered by heat and/or pressure, after they formed. Igneous means formed by fire, and igneous rocks were created at high temperatures, either in the volcanic heat of an eruption, or in the constant heat and pressure found deep underground.
Quartz, feldspar, and mica are some of the commonest minerals, contributing to many different rocks. Geologist believe there are about 1500 minerals, but only a few contribute to most rocks.
Silicate minerals are the commonest on earth. Almost all of the rock-forming minerals are of this type. They consist of a chemical combination of silicon and oxygen, with various other elements that change their form and color. Feldspar, mica and quartz all contain the element silicon.
The carbonate minerals store carbon. The commonest is calcite, which makes limestone and marble. It forms the stalactites and stalagmites in caves, and is the mineral in seashells. Most limestone formed in shallow seas, from the accumulation of shells, skeletons, coral, and algal debris. Limestone thus is sedimentary, formed from sediment. When it is compressed and heated it changes to marble, a metamorphic rock composed mostly of the mineral calcite.
Rare minerals found in rocks are sometimes considered gems. It is their rarity, as well as their beauty, that gives them value. (After all, common mica is quite beautiful.) Diamonds are composed of carbon, a very common element in a relatively rare form.
Ores are often valuable because the desirable minerals in them are easy to extract. Bauxite is the most important ore of aluminum. A mineral it contains, gibbsite, is easily separated from the bauxite, and then easily gives up its aluminum. Nevertheless, extracting aluminum from bauxite is quite costly compared to recycling.
Rocks are made of minerals. The minerals may be easily distinguished in certain rocks, such as granite. The particular combination of minerals in any rock gives it its form, characteristics, and use.
