How to Use Pictures of Rocks to Identify Their Type

Learn About the 3 Major Rock Types

Cloe-up view of woman stacking pebbles, at beach
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In geology, rocks can be classified and identified as one of three different types: igneous, sedimentary or metamorphic, and the Rock Type Identification Key can be used to help you best determine which type a particular rock belongs to. 

A variety of key factors including creating of the rock, materials found in the rock, and location of the rock help determine which of these three types the rock can be identified as belonging to.

Additional to these naturally occurring rocks, there are also a variety of categories for other rocks that don't fit into these categories such as man-made rocklike creations like bricks and concrete and interplanetary rocks with dubious origins.

Igneous Rocks

Igneous rocks were all once hot enough to melt and more often than not are formed and found around volcanic structures, and they can be identified by a number of factors related to this including being composed of primary minerals that are mostly black, white or gray and having a smooth or baked-looking surface.

The following 28 rocks are all considered igneous:

  1. Aa — jagged form of basalt lava
  2. Andesite — the typical intermediate-silica arc lava
  3. Anorthosite — plutonic rock of straight plagioclase feldspar
  4. Basalt — low-silica lava, the most common volcanic rock
  5. Diorite — plutonic rock between granite and gabbro
  6. Dunite — plutonic rock made of straight olivine
  1. Felsite — volcanic rock of the same composition as granite
  2. Gabbro — coarse-grained plutonic version of basalt
  3. Granite — familiar three-mineral rock essential to continents
  4. Granodiorite — plutonic rock between diorite and granite
  5. Komatite — rare and ancient ultramafic lava
  6. Lapillistone — volcanic rock formed of little ash balls
  1. Latite — dark, low-quartz lava, extrusive version of monzonite
  2. Obsidian — high-silica volcanic glass
  3. Pahoehoe — smooth-skinned flows of basalt lava
  4. Pegmatite — igneous rock with very large crystals
  5. Peridotite — dark, dense rock from Earth's mantle
  6. Perlite — hyrdous, lightweight obsidian or rhyolite
  7. Porphyry — any igneous rock with large mineral crystals in a fine matrix
  8. Pumice — light colored, frothy high-silica lava
  9. Pyroxenite — uncommon ultramafic rock from deep-sea crust
  10. Quartz Monzonite — much like granite, except short on quartz
  11. Rhyolite — high-silica white or red lava
  12. Scoria — dark, frothy low-silica lava
  13. Syenite — plutonic, alkalic granitic rock without quartz
  14. Tonalite — plutonic, felsic granitic rock without alkali feldspar
  15. Troctolite — gabbro without pyroxene
  16. Tuff — rock made from volcanic ash

Sedimentary Rocks

Sedimentary rocks are born cool at the Earth's surface, mostly underwater, and they usually consist of layers or strata; hence they are also called stratified rocks. The following list contains each commonly accepted type of sedimentary rock:

  1. Agate — semiprecious chert of infinite variety
  2. Alabaster — white gypsum stone long prized for sculpture
  3. Arkose — raw sandstone made from eroded granite
  1. Asphalt  — natural tar from petroleum seeps
  2. Banded Iron Formation — extremely ancient "tiger iron"
  3. Bauxite — highly leached surface rock and ore of aluminum
  4. Breccia — rock made from broken rocks
  5. Chert — common silica-rich rock type with much variety
  6. Claystone — stones made of clay
  7. Coal — the original fossil fuel
  8. Conglomerate — rocks made with extra-big grains
  9. Coquina — limestone composed of shell fragments
  10. Diamictite — rock made of poorly sorted land sediment
  11. Diatomite — rock made of microscopic plankton shells
  12. Dolomite — carbonate rock, a near-twin of limestone
  13. Graywacke — impure sandstone, also called wacke
  14. Gypsum Rock — soft rock formed early in evaporating waters 
  15. Limestone — rock composed of calcite, Earth's greatest store of carbon
  16. Peat — brown, vegetative precursor to coal
  17. Porcellanite — an earthy looking sub-chert
  1. Rock Salt — massive halite, the only edible stone
  2. Sandstone — where sand goes to and comes from
  3. Shale — thin-layered rock made from mud and clay
  4. Siltstone — made from sediment finer than sand
  5. Travertine — type of limestone precipitated in springs.

Metamorphic Rocks

Metamorphic rock formations occur when sedimentary and igneous rocks become changed, or metamorphosed, by conditions underground. The four main agents that metamorphose rocks are heat, pressure, fluids and strain. These agents can act and interact in an almost infinite variety of ways, and as a result, most of the thousands of rare minerals known to science occur in metamorphic rocks.

The following list presents a number of these metamorphic rocks:

  1. Amphibolite — high-grade rock, typically a hornblende schist
  2. Argillite — Low-grade rock, formerly claystone
  3. Blueschist—Rock made by high-grade metamorphism in subduction zones
  4. Cataclasite — Ground-up rock found in fault zones
  5. Eclogite — the most extreme metamorphic rock you can find
  6. Gneiss — banded and tough, it forms the lower crust
  7. Greenschist — rock made by low-grade metamorphism of various rock types
  8. Greenstone — dark rock made by metamorphism of basalt
  9. Hornfels — tough and fine-grained, it forms where igneous rocks cook it
  10. Marble — coarse-grained, metamorphosed limestone
  11. Migmatite — swirly-banded rock produced by extreme metamorphism
  12. Mylonite — milled and melted rock from deep in fault zones 
  13. Phyllite — shiny, colorful metamorphosed slate
  14. Quartzite — rugged metamorphosed sandstone
  15. Schist — finely striped rock made by metamorphism of mudstones
  1. Serpentinite — green, scaly metamorphosed ocean crust
  2. Slate — platy rock made by mild metamorphism of shale
  3. Soapstone — soft and carvable, hydrothermally altered lava

Other Rocks

While these account for most of the rock formations on Earth, there are still more types of rocks for rarer finds such as artificial rocks, concretions, fulgurite, geode, and meteorites — many of which fit into the larger categories above as well, though because of their origins or features are not considered the same rock type by geologists.

Other fun types of rocks include the Official State Rocks of the United States  —Twenty-five states have one — the fad from the 80s and 90s called Pet Rocks, and the Thunder Egg, which is a solid, agate-filled lump found in certain volcanic lands.

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Your Citation
Alden, Andrew. "How to Use Pictures of Rocks to Identify Their Type." ThoughtCo, Oct. 8, 2017, Alden, Andrew. (2017, October 8). How to Use Pictures of Rocks to Identify Their Type. Retrieved from Alden, Andrew. "How to Use Pictures of Rocks to Identify Their Type." ThoughtCo. (accessed January 17, 2018).