Category: Rocks

A porous limestone composed almost entirely of fossil debris. Article by: Hobart M. King, Ph.D., RPG Coquina: Coquina collected in Florida. This specimen measures approximately 9 centimeters across. Public Domain photo by Mark A. Wilson of the Department of Geology, The College of Wooster. Click to enlarge.

An extrusive igneous rock with a very high silica content. Article by: Hobart M. King, Ph.D., RPG Rhyolite: A pink specimen of rhyolite with numerous very tiny vugs with some evidence of flow structures. The specimen shown here is about two inches across. Igneous rock composition chart: This chart shows that rhyolite is typically composed of orthoclase, quartz, plagioclase, micas, and amphiboles.

When molten rock material cools, a wide variety of geologic features can form. Article by: Terri Cook, M.S. Although all igneous rocks form from the solidification of molten material, they can have very different appearances and characteristics depending upon the composition of the original material and where exactly it cooled.

The metamorphic rock composed almost entirely of quartz. Article by: Hobart M. King, Ph.D., RPG Quartzite: A specimen of quartzite showing its conchoidal fracture and granular texture. The specimen shown is about two inches (five centimeters) across. What is Quartzite? Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz.

Article by: Hobart M. King, Ph.D., RPG A sliding rock that has left a long track across the surface of Racetrack Playa. Some tracks are hundreds of feet long! (See below for several more sliding rock photos.) Image copyright iStockphoto / Steve Geer. The Sliding Rocks Mystery One of the most interesting mysteries of Death Valley National Park is the sliding rocks at Racetrack Playa (a playa is a dry lake bed).

Photos of Common Intrusive and Extrusive Igneous Rock Types Article by: Hobart M. King, Ph.D., RPG Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite. The specimen shown is about two inches (five centimeters) across.

Photos of Common Clastic, Chemical, and Organic Sedimentary Rock Types. Article by: Hobart M. King, Ph.D., RPG Breccia is a clastic sedimentary rock that is composed of large (over two-millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together.

It's far more than a black rock! Article by: Hobart M. King, Ph.D., RPG Splint coal: This is a highly magnified view of splint coal in transmitted light. The large yellow object in the center of this image is a spore - a reproductive cell of the coal-forming vegetation. It is about two millimeters long.

Photos of Common Foliated and Non-Foliated Metamorphic Rock Types Article by: Hobart M. King, Ph.D., RPG Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of hornblende (amphibole) and plagioclase, usually with very little quartz.

Shale is the most abundant sedimentary rock and is in sedimentary basins worldwide. Article by: Hobart M. King, Ph.D., RPG Shale: Shale breaks into thin pieces with sharp edges. It occurs in a wide range of colors that include red, brown, green, gray, and black. It is the most common sedimentary rock and is found in sedimentary basins worldwide.

What is Granite? What is Granite Used For? Article by: Hobart M. King, Ph.D., RPG Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite.

What is Soapstone? How Does it Form? How is it Used? Article by: Hobart M. King, Ph.D., RPG Soapstone: A metamorphic rock that consists primarily of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, pyroxenes, and carbonates. It is a soft, dense, heat-resistant rock that has a high specific heat capacity.

The sedimentary rock used as a filter, absorbent, filler, abrasive, and more. Article by: Hobart M. King, Ph.D., RPG Diatomite as a beer filter: Diatomite has a very small particle size, a high porosity, and is relatively inert. That makes it an excellent material for use as a filter. Much of the beer brewed in the United States is filtered through crushed diatomite, known as diatomaceous earth.

A hard, tough material, used by humans to make tools for millions of years Article by: Hobart M. King, Ph.D., RPG Flint nodule: Flint is a variety of microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses, and less frequently as a layered deposit. It breaks consistently with a conchoidal fracture and was one of the first materials used to make tools by early people.

Sand can be one of the most interesting materials when viewed through a microscope! Gobi Desert sand: Highly rounded sand grains from the Gobi Desert of Mongolia. Wind-blown sand sustains repeated tiny impacts as it bounces along Earth's surface. These impacts gradually abrade sharp protrusions from the grains and give their surface a "frosted" luster.

A foliated metamorphic rock composed mainly of tiny mica grains in parallel alignment. Article by: Hobart M. King, Ph.D., RPG Phyllite: A specimen of phyllite that exhibits the lustrous and wrinkled surface common to this rock type. What Is Phyllite? Phyllite is a foliated metamorphic rock that has been subjected to low levels of heat, pressure and chemical activity.

Used in monuments, crushed stone, cosmetics, pharmaceuticals, and more. Article by: Hobart M. King, Ph.D., RPG Taj Mahal The Taj Mahal is one of the most beautiful and famous buildings in the world. It was built between 1632 and 1653 as a mausoleum for Mumtaz Mahal, the third wife of Mughal emperor Shah Jahan.

Article by: Hobart M. King, Ph.D., RPG Andesite: The specimen shown is about two inches (five centimeters) across and has a porphyritic texture. Igneous rock composition chart: This chart shows that andesite is typically composed of plagioclase, amphiboles, and micas; sometimes with minor amounts of pyroxenes, quartz, or orthoclase.

Article by: Hobart M. King, Ph.D., RPG Gneiss: The specimen shown is about two inches (five centimeters) across. It is easy to see the "salt and pepper" banding of this rock. What is Gneiss? Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture.

A group of ultramafic rocks, including Kimberlite. They sometimes contain chromite or diamonds. Article by: Hobart M. King, Ph.D., RPG Kimberlite with diamond: Kimberlite, the rock that is found in many diamond pipes, is a variety of peridotite. The specimen above is a piece of kimberlite with numerous visible grains of phlogopite and a six millimeter octahedral diamond crystal of about 1.