Igneous rocks are formed when molten rock (magma) cools and solidifies, with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. This magma can be derived from either the Earth’s mantle or pre-existing rocks made molten by extreme temperature and pressure changes. Over 700 types of igneous rocks have been described, most of them formed beneath the surface of the Earth’s crust. The word “igneous” is derived from the Latin ignis, meaning “fire”.

Granite is a common and widely-occurring group of intrusive felsic igneous rocks that form at great depths and pressures under continents. Granite consists of orthoclase and plagioclase feldspars, quartz, hornblende, biotite, muscovite and minor accessory minerals such as magnetite, garnet, zircon and apatite.

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Marble is metamorphosed limestone, composed of fairly pure calcite (a crystalline form of calcium carbonate, CaCO3). It is extensively used for sculpture, as an building material, and in many other applications.

Limestone is a sedimentary rock composed of the mineral calcite (calcium carbonate). The primary source of this calcite is usually marine organisms.

Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Limestone makes up approximately 10 percent of the total volume of all sedimentary rocks.

Sandstone is an arenaceous sedimentary rock composed mainly of feldspar and quartz and varies in colour (in a similar way to sand), through grey, yellow, red, and white. Since sandstones often form highly visible cliffs and other rock formations, certain colors of sandstone may be strongly identified with certain regions. For instance, much of the American West is well-known for its red sandstones.

Sandstones are often relatively soft and easy to work which therefore make them a common building and paving material.

Slate is a fine-grained homogeneous sedimentary rock composed of clay or volcanic ash which has been metamorphosed (foliated) in layers (bedded deposits). Slate can be made into roofing shingles (‘roofing slates’ in the UK) because it has two lines of breakability: cleavage and grain. This makes it possible to split slate into thin sheets.

Quartzite is a hard, metamorphic rock which was originally sandstone. Through heating and pressure usually related to tectonic compression within orogenic belts, the original quartz sand grains and quartz silica cement were fused into one. Pure quartzite is usually white to grey. Quartzites often occur in various shades of pink and red due to varying amounts of iron oxide. Other colors are due to impurities of minor amounts of other minerals.

Orthoquartzite is a very pure quartz sandstone composed of usually well rounded quartz grains cemented by silica. Orthoquartzite is often 99% SiO2 with only very minor amounts of iron oxide and trace resistant minerals such as zircon, rutile and magnetite.

Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. The nearly pure silica content of the rock provides little to form soil from and therefor the quartzite ridges are often bare or covered only with a very thin soil and little vegetation.

Because of its hardness (about 7 on Mohs’ scale of mineral hardness), crushed quartzite is often used as railway ballast.

Travertine is a white concretionary form of calcium carbonate that is usually hard and semicrystalline. It is deposited from the water of mineral springs (especially hot springs) or streams holding lime in solution. Extensive deposits exist at Tivoli, Italy, near Rome. The largest building in the world constructed largely of travertine is the Colosseum in Rome, Italy.