Simple definitions for geology, fossil, mineral, and rock collecting terms used across the site.
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1.
Agate
A variety of chalcedony that often forms in layers or bands. Agate is made of microscopic quartz crystals.
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Agatized
A fossil or rock that has been replaced or filled by chalcedony, agate, or other silica-rich material.
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3.
Augen
A German word meaning "eyes." In geology, augen are eye-shaped mineral grains, often feldspar, surrounded by a stretched or folded rock matrix.
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Chalcedony
A microcrystalline form of quartz/silica. Its crystals are too small to see clearly without magnification.
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Chert
A hard, fine-grained rock made mostly of microscopic quartz.
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Conglomerate
A sedimentary rock made of rounded pebbles or larger rock fragments held together by a finer matrix of sand, silt, or mud.
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Crinoid
A marine animal related to sea stars and sea urchins. Fossil crinoid stems often look like small stacked discs or beads.
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Devonian Period
A geologic time period from about 419 to 359 million years ago. Many Michigan coral fossils, including Petoskey stones, are Devonian in age.
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Fossil
Preserved evidence of ancient life, such as shells, bones, tracks, burrows, or the mineralized remains of plants and animals.
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Genus
A scientific classification group above species. In a scientific name like Favosites sp., Favosites is the genus.
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Glacial Erratic
A rock carried away from its original source by a glacier and left behind when the ice melted.
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Gneiss
A high-grade metamorphic rock with visible bands or layers of minerals.
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Greenschist
A greenish metamorphic rock that commonly forms from basaltic or other mafic rocks under low- to medium-grade metamorphic conditions.
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14.
Jasper
An opaque, usually red, yellow, brown, or green variety of microcrystalline silica. Red jasper commonly gets its color from iron minerals.
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15.
Limestone
A sedimentary rock made mostly of calcium carbonate. Many limestones form from shells, coral, and other marine material.
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Matrix
The surrounding rock material that holds larger grains, pebbles, crystals, or fossils in place.
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Metamorphic Rock
A rock that has been changed by heat, pressure, mineral-rich fluids, or some combination of those conditions.
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Mineral
A naturally occurring solid material with a specific chemical composition and crystal structure.
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Nodule
A rounded lump or mass that forms inside sediment or sedimentary rock.
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20.
Petoskey Stone
A fossil colonial rugose coral from Michigan, commonly identified as Hexagonaria cf. percarinata. Petoskey stones are Michigan's official state stone.
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21.
Puddingstone
A conglomerate or metaconglomerate with rounded pebbles in a contrasting matrix. Michigan puddingstones commonly have a white quartzite base with red jasper and other dark inclusions.
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22.
Quartz
A common mineral made of silicon dioxide. It is hard, glassy, and found in many igneous, sedimentary, and metamorphic rocks.
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Quartzite
A hard metamorphic rock formed when quartz-rich sandstone is changed by heat and pressure.
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Rugose Coral
An extinct type of Paleozoic coral. Some rugose corals were solitary, while others formed colonies.
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25.
Sandstone
A sedimentary rock made mostly of sand-sized grains that have been compacted and cemented together.
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Sedimentary Rock
A rock formed from sediment, pieces of older rocks, minerals, or once-living organisms that accumulated at Earth's surface.
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Septarian Nodule
A concretion or nodule with internal cracks that later filled with minerals such as calcite, quartz, or other silica minerals.
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28.
Silica
A chemical compound made of silicon and oxygen. Quartz, chalcedony, agate, jasper, and chert are all silica-rich materials.
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Silicification
The process where silica-rich fluids fill spaces in a rock or replace original material with silica.
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30.
Silurian Period
A geologic time period from about 444 to 419 million years ago. Many chain corals and tabulate corals are Silurian in age.
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31.
Species
A more specific scientific classification below genus. In Hexagonaria cf. percarinata, percarinata is the species name.
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sp.
An abbreviation used when the genus is known but the exact species is uncertain or not identified. Example: Favosites sp.
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33.
cf.
An abbreviation meaning "compare with." It is used when a specimen looks similar to a species, but the identification is not completely certain. Example: Hexagonaria cf. percarinata.
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34.
Tabulate Coral
An extinct group of colonial Paleozoic corals. Favosites and Halysites are examples of tabulate corals.