Arizona Rocks

The official seal of the State of Arizona contains the motto “Ditat Deus,” meaning “God Enriches.” This is certainly true in regard to Arizona’s abundance of mineral resources. Rocks and minerals give the Southwest landscape its color, texture, and shape. Arizona is like a geologic wonderland filled with a wide variety of rock formations and mineral treasures.

Minerals are inorganic substances made of one or more chemical elements. Minerals exhibit many differences in form, color, and other properties. Metal ores, such as gold and silver, are minerals. Crystalline substances, like salt and quartz, are minerals. Gem stones, highly prized for their beauty, are a special subgroup of minerals. Minerals are the building blocks of rocks.

Rocks are composed of either one mineral or a combination of minerals. Igneous rocks are formed by volcanic activity, such as when molten magma cools and hardens, either underground (resulting in coarse grains) or on the surface (producing fine grains). Sedimentary rocks are made from sediments and small pieces of rocks, sand, or shells. Metamorphic rocks are created when existing rocks are reshaped and transformed by extreme heat and great pressure.

Minerals

Agate- A striped, banded, or variegated semiprecious form of quartz. Petrified wood is usually an agatized wood.

Amethyst- A crystalline type of quartz colored purple by manganese.

Calcite- Calcite is a common mineral made of calcium. Pure calcite is white but crystals may be clear. It often forms around springs and in caves.

Chalcedony- A smooth frosty white or gray translucent form of quartz that fills cracks, lines cavities, and forms crusts.

Chert- A brittle form of quartz that has sharp edges, used for making arrowheads. Opaque brown, yellow, or gray in color. Black chert is called flint.

Copper- May be found in its native metallic form or in greenish ores such as chrysocolla, malachite and azurite.

Feldspar- A pale pink mineral commonly found in granite.

Fluorite- Calcium fluoride occuring as glassy purple or yellow chunks or crystals, either cube-shaped or eight-sided.

Galena- A mineral that contains lead and forms cube-shaped crystals.

Gold- A precious metal, often associated with veins of quartz.

Gypsum- Calcium sulfate occurs naturally in soft white layers, or in rounded shapes called “desert roses.”

Halite- Naturally occurring sodium chloride (rock salt).

Hematite- Iron ore, dense and colored dark red or dark gray.

Jasper- A pretty, opaque quartz usually colored red or reddish brown.

Magnetite- A grayish black iron ore that is naturally magnetic and can be picked up easily with a magnet. Lodestone is a form of magnetite.

Manganese- Dark purple or steel gray, may have fern-like patterns (dendrites), gives amethyst its purple color.

Mica- A colorless or black silicate mineral that comes in thin, flexible sheets. Found in granite and other igneous rocks, it also forms in metamorphic rocks.

Pyrite- A metallic mineral made of iron and sulfur, brassy yellow in color and forming cubic crystals, also known as fool’s gold.

Olivine- An olive green mineral common in igneous rocks.

Quartz- The most common mineral, a form of silica. It is clear or colored and may or may not have crystals. Quartz can be milky, smoky, or rosy in appearance. Other types of quartz include: amethyst, tiger eye, agate, jasper, flint, chert, chalcedony.

Silver- A precious metal, often associated with ores of lead and zinc.

Tiger Eye- A type of quartz that has mineral fibers trapped inside.

Turquoise- A semiprecious gem made of copper and aluminum phosphate.

Igneous Rocks

Andesite- A fine-grained medium gray rock, intermediate between rhyolite and basalt.

Basalt- The most common volcanic lava rock. Fine-grained, dark brown or greenish black. In arid regions, exposed surfaces get a white limy encrustation.

Diorite- Medium-grained rock with a salt and pepper appearance.

Gabbro- A dark gray or greenish coarse-grained rock similar to granite. It is also sometimes called diabase.

Granite- A common coarse-grained speckled rock of various light colors. It mainly contains minerals of quartz, feldspar, and mica.

Obsidian- Volcanic glass, usually shiny and black.

Porphyry- A fine-grained basalt matrix embedded with larger minerals.

Pumice- A highly porous lightweight lava rock that floats.

Rhyolite- A fine-grained light-colored gray or pink rock.

Vesicular Basalt- Basalt containing holes that were once gas bubbles. Sometimes these holes become filled with calcite.

Sedimentary Rocks

Conglomerate- Rounded pebbles of various sizes stuck in a cement of hardened clay, silica, calcium carbonate, or iron oxide.

Breccia- A type of conglomerate consisting of angular stone fragments.

Limestone- Consists mainly of calcium carbonate or calcite. Chalk is a finely compacted form of limestone.

Sandstone- A rock made of quartz sand grains held together by silica, lime, or iron oxide.

Shale- A fine-grained rock made of hardened clay or layers of mud.

Siltstone- A fine-grained rock made of sand grains mixed with clay.

Metamorphic Rocks

Gneiss- A coarse-grained rock that looks like granite with a banded appearance.

Marble- A hard limestone, pure white or in varied patterns.

Quartzite- Dense sandstone with quartz grains tightly compacted.

Schist- Sparkly rocks with grains of flaky minerals such as mica.

Slate- A hard shale that splits easily into smooth, flat sheets.

Books

“Gem Trails of Arizona,” by James Mitchell.

“Gems and Minerals of Arizona,” by Sharon and Bill Panczner.

“Geology of Arizona,” by J. Dale Nations.

“Hiking Arizona’s Geology,” by Ivo Lucchitta.

“Mineralogy of Arizona,” by John W. Anthony, et al.

“Minerals of Arizona,” by Neil Bearce.

“Roadside Geology of Arizona,” by Halka Chronic.

“Rockhounding Arizona,” by Gerry Blair.

Websites

www.geocities.com/bccrockclub (Black Canyon City Rock Club.)

www.admmr.state.az.us (Arizona Department of Mines and Mineral Resources educational information.)

www.desertmuseum.org/visit/exhibits_earthsciences.html (The Arizona-Sonora Desert Museum has an extensive collection of regional minerals and geology exhibits.)

www.geo.arizona.edu/minmus (University of Arizona Mineral Museum.)

www.azcu.org/educators/index.html (Online educational resources from the Arizona Mining Association.)

www.azgs.az.gov/minerals.htm (Arizona’s mineral resources, from the Arizona Geological Survey.)

www.azminfun.com (Official website of the Arizona Mineral and Mining Museum Foundation.)

http://reynolds.asu.edu/azgeomap/azgeomap_home.htm (Clickable geologic map of Arizona, with photos.)

www.desertusa.com/Thingstodo/geo/geology.html (Desert Geology: Rocks, Gems, and Minerals.)

These pages are a continuous work in progress.
Copyright © 2000- by Teri Ann Berg Olsen