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Category: Learn Minerals, Rocks and Crystalz

  • Coquina

    Coquina is a type of limestone composed of calcium carbonate shells, shell fragments, and other sand-sized fossil debris. It forms in the shallow waters of coastal areas with a tropical or subtropical climate. The particles are weakly cemented together, and therefore coquina is a very porous material that can function as an aquifer or a reservoir for oil and natural gas.…

  • Chert

    Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It occurs as nodules and concretionary masses, and less frequently as a layered deposit. It breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons.…

  • Coal

    Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel. The specimen shown above is about two inches (five centimeters) across.

  • Chalk

    Chalk is a type of limestone made up of the microscopic calcium carbonate shells of marine organisms. Chalk is soft, friable, porous, and effervesces vigorously in contact with hydrochloric acid. Because it is very porous, subsurface chalk units can serve as reservoirs for oil and natural gas.

  • Caliche

    Caliche is found in arid or semiarid climates around the world. It forms when rock fragments and sediment particles are cemented together by mineral matter, usually calcium carbonate, that precipitates and hardens between the particles. Over time, caliche can become very dense and durable, which explains why it is also called hardpan, calcrete, and duricrust. Some…

  • What Are Sedimentary Rocks?

    Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks. Clastic sedimentary rocks form from the accumulation and lithification of mechanical weathering debris. Examples include: breccia, conglomerate, sandstone, siltstone, and shale. Chemical sedimentary rocks form when dissolved materials preciptate from solution. Examples include: chert, some dolomites, flint, iron ore, limestones, and rock salt. Organic sedimentary rocks form from the accumulation of plant or…

  • Schist

    Schist is a metamorphic rock with well-developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a “chlorite schist” because it contains a significant amount of chlorite. It is about two inches (five centimeters)…

  • Quartzite

    Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz. The specimen above is about two inches (five centimeters) across.

  • Phyllite

    Phyllite is a foliated metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist. The specimen shown above is about two inches (five centimeters) across.

  • Novaculite

    Novaculite is a dense, hard, fine-grained, siliceous rock that breaks with a conchoidal fracture. It forms from sediments deposited in marine environments where organisms such as diatoms (single-celled algae that secrete a hard shell composed of silicon dioxide) are abundant in the water. The specimen shown above is about three inches across.