Rocks are fundamental components of our planet, forming the Earth’s crust, from towering mountains to the deep ocean floor. Ubiquitous yet often hidden beneath soil, rocks are composed of minerals like quartz, feldspar, calcite, and mica. While some rocks are made of a single mineral, most are a combination, similar to how a car is made of steel, glass, and plastic – where minerals are the steel, glass, and plastic, and the rock is the car.
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Exploring the Main Types of Rocks
There are three primary categories of rocks, each formed through distinct geological processes:
Igneous Rocks: Born from fire, igneous rocks originate from the cooling and crystallization of magma beneath the Earth’s surface or lava erupting onto it. Magma that reaches the surface is known as lava. Slow cooling underground results in larger, visible crystals, while rapid surface cooling leads to minuscule crystals, often requiring magnification to see. In instances of extremely rapid cooling, obsidian, a black, glassy rock, can form.
Sedimentary Rocks: These rocks are created from sediment, which are particles eroded from pre-existing rocks. Sediment includes sand, silt, and clay, categorized by particle size, with sand being the coarsest and clay the finest. Gravel denotes sand mixed with pebbles. Sediment transforms into rock through compaction under the weight of overlying layers and cementation by minerals dissolved in water. Often, both processes occur simultaneously.
Metamorphic Rocks: Transformation is key to metamorphic rock formation. These rocks are formed by intense heat, pressure, or a combination of both. Pressure can stem from deep burial within the Earth’s crust or tectonic plate collisions. Temperature increases with depth, so deep burial implies high temperatures. Additionally, rising magma can elevate temperatures in the surrounding crust, baking adjacent rocks. Hot liquids and gases from magma can also induce chemical alterations in nearby rocks.
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The Continuous Rock Cycle
Rocks, like mountains, are not permanent fixtures. Weathering, water erosion, and glacial activity gradually break them down into sediment—sand, silt, and clay. As sediment accumulates and buries deeper, compression and mineral cementation transform it into sedimentary rock. Intense pressure or heat can further convert sedimentary rock into metamorphic rock. At sufficient depths, rocks melt, becoming magma. When this magma ascends and cools, it crystallizes to form igneous rocks, completing the continuous Rock Cycle.
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Key Minerals in Rock Formation
The number of common rock-forming minerals is relatively small. With practice, you can learn to identify most of them. Here are descriptions of some key minerals as they appear in rocks:
Quartz: As the last mineral to crystallize, quartz in igneous rocks lacks a defined shape. It typically appears gray in igneous rocks, gray, white, yellow, or red in sedimentary rocks, and gray or white in metamorphic rocks. Its luster is glassy, sometimes waxy.
Potassic Feldspars*: (microcline, orthoclase) These feldspars are pink, tan, or sometimes white. In igneous rocks, they exhibit flat, shiny faces and often appear as blocky, nearly rectangular grains, resembling china.
Plagioclase Feldspars*: (albite, labradorite) Similar in appearance to potassic feldspars but range from white to dark gray, occasionally black. Some may display flashes of blue or green.
Micas*: (muscovite, biotite, phlogopite) Micas are characterized by very thin, easily cleaved layers. They appear as flakes or layers in rocks. Muscovite is silvery to brown, biotite is black, and phlogopite is reddish-brown, sometimes found in marble.
Chlorite*: Resembles mica but with less thin flakes that are harder to peel apart. It is medium to dark green, sometimes almost black with a greenish tint.
Hornblende: Dark green to black, hornblende displays nearly flat, shiny faces in roughly rectangular or elongated, needle-like crystals within rocks. It’s common in dark metamorphic rocks and sometimes in igneous rocks.
Actinolite and Tremolite: These usually occur as long, thin blades or needle-like crystals. Actinolite is dark green, while tremolite ranges from white to gray. Crystals may be parallel or radiate from a central point. Actinolite is typically found in schists and gneisses; tremolite may occur in marble.
Olivine*: In rocks, olivine is olive green to greenish-yellow, appearing as rounded, almost sugary grains, primarily in dark igneous rocks.
Calcite and Dolomite: Typically white but can be colored by impurities. Crystal grains show flat, shiny faces, often parallelogram-shaped. Both are soft and easily scratched. Powdered calcite fizzes in white vinegar, while dolomite does not. These minerals are found in limestone or dolostone (dolostone is the rock, dolomite is the mineral) and marble.
Note: Names marked with an asterisk (*) represent groups of related minerals.
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Building Your Rock Collection
Rock collecting is more accessible than mineral collecting because rocks are prevalent. To start a collection, seek freshly broken pieces from a ledge—an exposed bedrock outcrop. Avoid collecting rocks buried in soil or rounded by water, as their origins and composition are harder to discern.
Rock Collecting Best Practices:
- Gather clean, fresh specimens.
- Create a label noting the rock name and collection location.
- Assign a unique number to each rock.
- Record the name, location, and number in a notebook.
- Paint a small white rectangle on each rock and write its number on it for easy identification.
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Safety First When Collecting Rocks
Prioritize safety during rock collecting:
- Always wear safety glasses or goggles when breaking rocks to protect your eyes from flying fragments.
- Use hammers specifically designed for rock breaking. Never use a carpenter’s claw hammer, as hard rocks can shatter it, sending sharp steel splinters.
- Avoid climbing dangerous ledges or quarry walls. Quarry walls are unstable and can collapse.
- Never enter mine tunnels. They are extremely hazardous.
- Ideally, collect rocks with an adult for added safety and guidance.
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Navigating The Rock Key for Identification
This Rock Key uses hyperlinks for easy navigation. Blue, underlined links allow you to jump between sections. Yes and No choices are also links, directing you to the next relevant question in the identification process. The numbers preceding each step are for paper-based use and are not crucial for web browser navigation.
To retrace your steps, use the Go Back links or your browser’s back button.
Mineral descriptions often include a “Compare To:” section with links to similar-looking rocks for comparison.
Understanding Key Terms for Rock Identification:
Crystals: Minerals form crystals when they grow freely in nature, like the standalone quartz crystal. In rocks, crystals grow in close proximity, resulting in straight edges and often flat, shiny faces that reflect light.
Grains: Non-crystal grains in rocks lack flat, shiny faces and are rounded, like sand grains, or jagged, like broken rock fragments.
Grain Size: Refers to the size of crystal grains or fragments in rocks:
- Coarse Grained: Most of the rock consists of grains as large as or larger than rice.
- Medium Grained: Individual grains are visible without magnification but are mostly smaller than rice.
- Fine Grained: Individual grains are not visible without magnification (or a microscope).
Layers: Layers manifest in rocks in various ways:
- Ribbon-like Layers: Bands of different colored minerals, often in contrasting colors like black and white, green and white, or black and tan/pink, as seen in gneiss.
- Mica-like Layers: Thin layers of mica or chlorite surrounding lens-shaped masses of feldspar or quartz, common in schists.
- Particle Layers: Variations in sand grain size or color shades within sandstones, often due to sorting by water or wind.
- Thin Cleavage Layers: Very thin, straight layers with flat, smooth top and bottom surfaces, characteristic of slate.
| Ribbon-like Layers | Mica-like Layers | Particle Layers | Thin Cleavage Layers |
|---|---|---|---|
| in Gneiss | in Schist | in Sandstone | in Slate |
Gas Bubbles: These appear as rounded or elongated holes in rocks. In pumice, bubbles can be tiny to match-head size, creating a sponge-like or glassy froth. In scoria or vesicular basalt, bubbles are larger, often pea-sized, resembling small pockets.
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| The Rock Key |
|---|
| 1. Is the rock made of crystal grains? (Does it have many flat, shiny faces – tiny to small – reflecting light like mirrors? Magnification may be needed.) The rock is made of crystal grains with flat shiny surfaces. . . Go to 2 There are no (or not many) shiny, flat, crystal grains. . . Go to 3 [ Return to Rock Key Table of Contents ] |
| 2. Does the rock have both layers and crystal grains? (Carefully check for layers, especially along edges. Magnification may be needed.) The rock has both layers and crystals. . . Go to 4 The rock has crystals, but no layers. . . Go to 5 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 3. Does the rock have layers but not crystal grains? (Carefully check for layers, especially along edges. Magnification may be needed.) The rock has layers, and crystal grains are not visible. . . Go to 11 The rock has no layers, and crystal grains are not visible. . . Go to 12 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 4. Do the layers look like ribbons or bands of minerals running through the rock; and is the rock kind of blocky? (Bands may be straight or wavy. Rock breaks into blocky chunks, not along layers.) The rock has crystals, ribbon-like mineral bands, and is blocky. It is. . . Go to 23 The rock has crystals and thin layers unlike mineral ribbons. It breaks along layers. It is. . . Go to 24 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 5. Is the entire rock mostly light colored, compared to other rocks? (Assess the whole rock color, not just mineral grains.) The rock is mostly light colored or light gray mineral grains. . . Go to 6 The rock is mostly medium gray to very dark colored minerals. . . Go to 7 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 6. Can you scratch glass with the rock? (If yes, it’s hard. If no, it’s soft.) (Safety note: glass flat on desk, not in hand. Gently press rock point against glass and pull 2cm. Check for scratch. Do not hit glass.) The rock scratches glass. Crystals, no layers. . . Go to 9 The rock does not scratch glass. Crystals, no layers. It is. . . Go to 25 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 7. Is the rock mostly light or medium gray, not very dark gray or black? The rock is mostly light to medium gray, has crystal grains, and is not layered. It is. . . Go to 31 The rock is mostly very dark gray or black. . . Go to 8 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 8. Can you see crystal grains in most or all of the rock without using a magnifier? The rock is coarse or medium grained, has crystals, and no layers. . . Go to 10 The rock is fine grained, has crystals, and no layers. It is. . . Go to 32 [ Go Back ] ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 9. Can you see crystal grains in most or all of the rock without using a magnifier? The rock is mostly crystal grains. Medium or coarse grained, no layers, light colored. It is. . . Go to 30 The rock is mostly fine grained, crystal grains, no layers, light colored. It is. . . Go to 29 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 10. Is the rock coarse grained? (Coarse grained: most crystals as large as or larger than rice. Medium grained: visible crystals without magnifier, smaller than rice.) The rock is made of coarse crystal grains. No layers, dark colored. It is. . . Go to 34 The rock is made of medium crystal grains. No layers, dark colored. It is. . . Go to 33 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 11. Using a steel nail point, can you scrape sand grains off? (Hold rock over clean paper, scrape hard with nail point. Rub finger on paper. Feel sand grains?) The rock has layers. Made of sand grains. It is. . . Go to 38 The rock has layers and is not made of sand grains. . . Go to 13 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 12. Does the rock have gas bubbles? (Sponge-like look. Rounded holes or glassy bubbles. Tiny {pinhead}, small, or large {pea} ) The rock has gas bubbles. . .Go to 15 The rock has no gas bubbles. . .Go to 17 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 13. Does the rock look mostly one mineral with many thin flat layers? (Layers less than 2mm thick, mostly thinner. Not thick layers) The rock has many thin flat layers, seems one mineral, usually no visible crystals. It is. . . Go to 27 The rock is mostly one mineral, but layers are thicker (usually >4mm). . .Go to 14 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 14. Is the rock definitely green and slippery? The rock is mostly green and slippery. It is. . . Go to 28 The rock is not green and slippery. It is. . . Go to 39 [ Go Back [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 15. Is the rock light and mostly light colored (probably gray)? Full of gas bubbles, light weight, light colored. It is. . . Go to 35 The rock is heavy, dark colored, some gas bubbles, mostly larger. . . Go to 16 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 16. Is the rock dark colored, glassy, with gas bubbles? (Jagged or sharp points? ) Dark colored, glassy, gas bubbles. It is. . . Go to 36 Gray or black, few gas pockets, no layers, not glassy. It is. . . Go to 32 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 17. Does the rock look like black glass with no bubbles? (White “snowflakes” or reddish bands possible) Looks like black glass. It is . . Go to 37 Does not look like black glass. . . Go to 18 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 18. Using a steel nail point, sand can be scraped off. (Scrape rock over clean paper with steel nail. Feel sand on paper?) Sand can be scraped off. . . Go to 19 Sand cannot be scraped off. . . Go to 20 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 19. Does the rock contain sand and larger rock pieces or pebbles? Composed of sand and pebbles or larger rock pieces. . . Go to 22 Made of sand, but not pebbles or larger pieces. It is. . . Go to 38 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 20. Can the rock scratch glass? (Safety note: glass flat on desk, not in hand. Gently press rock point against glass, pull 2cm. Check for scratch) Rock scratches glass, not made of sand. . . Go to 21 Rock does not scratch glass. Not visible crystals. It is. . . Go to 40 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 21. Is the rock white, yellowish, tan, or reddish? The rock is.. . . Go to 26 The rock is black or gray. It is. . . Go to 32 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
| 22. Are larger rock pieces (mixed with sand) rounded pebbles, not blocky or jagged? Larger pieces are rounded pebbles. It is. . . Go to 41 Larger pieces are jagged and blocky. It is. . . Go to 42 [ Go Back ] [ Go to Beginning of Key ] [ Return to Rock Key Table of Contents ] |
Metamorphic Rocks
23.
| GNEISS (nice) |
|---|
| Rock Type: Metamorphic |
| Appearance: Usually light-colored but can be dark. Features mineral ribbons or stripes. Typically coarse-grained. Breaks into blocky pieces, not along layers. Crystals are aligned in layers, unlike randomly arranged granite crystals. Hard and tough. |
| Mineral Composition: Almost always: feldspars, quartz, and mica. Sometimes: kyanite, garnet, hornblende, tourmaline, magnetite, and more. |
| Formation: Often from schist, another metamorphic rock derived from fine-grained sedimentary rock (often shale). Can also form from igneous rocks, especially granite. Typically formed under high pressure from tectonic plate movement. |
| Compare To: schist granite |
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24.
| SCHIST (sh-ist) |
|---|
| Rock Type: Metamorphic |
| Appearance: Top/bottom layers usually silvery, green, brown, or black mica, or green to very dark green chlorite. Micas often in small flaky crystals. Thin layers, often with lens-like quartz layers between mica layers. Layers may be wavy. Grain size medium to coarse. Splits easily along mica layers, unlike gneiss. |
| Mineral Composition: Quartz, feldspar, mica (muscovite, biotite). Sometimes: chlorite, garnet, hornblende, actinolite, kyanite, magnetite, pyrite, staurolite, tourmaline, and more. |
| Formation: Typically from shales derived from clay or sandy clay, sometimes with lime, or volcanic rocks/sediments. Often forms when ocean floor plates subduct under continents, crushing seafloor rocks into schists. |
| Compare To: gneiss, shale, slate, serpentinite |
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25.
| MARBLE (mar’-bul) |
|---|
| Rock Type: Metamorphic |
| Appearance: Often pure white. May have gray, green, tan, or red streaks or patches. Fine to very coarse-grained, crystals usually visible. Soft; does not scratch glass (quartzite may look similar but scratches glass easily). Powdered marble often fizzes with white vinegar (dolomitic marble may not fizz). |
| Mineral Composition: Calcite or dolomite (dolomitic marble). Sometimes: graphite, pyrite, mica, tremolite, and a few others. |
| Formation: Metamorphosis of limestones. |
| Compare To: quartzite , limestone |
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26.
| QUARTZITE (kwart’-zite) |
|---|
| Rock Type: Metamorphic |
| Mineral Composition: Quartz. Sometimes, small amounts of: mica, feldspar, magnetite, pyrite, ilmenite, garnet, and others. |
| Appearance: Pure quartzite is white. May be yellowish to reddish with iron minerals, or rarely black with magnetite. Sand grains may be visible with magnification. Breaks through grains, not around them (sandstone breaks around grains). Often shows lighter flakes on broken surfaces. Very hard; easily scratches glass (unlike marble). |
| Formation: Mostly metamorphosed sandstone. |
| Compare To: marble, sandstone |
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27.
| SLATE (sl-ate) |
|---|
| Rock Type: Metamorphic |
| Mineral Composition: Micas, feldspars, quartz (grains too small to see without microscope). Sometimes pyrite. |
| Appearance: Black, gray, brownish-red, bluish-gray, or greenish-gray. Very fine-grained, thin, smooth, flat layers. Splits easily into thin flat pieces (unlike shale). Often scratches glass with slight difficulty. |
| Formation: Usually from clay sediments or shale heated and pressurized by plate collisions. Forms at lower pressures and temperatures than schist. |
| Compare To: shale, schist, serpentinite |
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28.
| SERPENTINITE (Sir’-pen-tin-ite) |
|---|
| Rock Type: Metamorphic |
| Appearance: Very slippery feel. More broken rock than layered. “Layers” are flat green rock plates, thin to >2cm thick. Usually green to grayish-green. Plates may have scratch-like grooves. Can be dull or nearly glassy. Dull serpentine: fine to coarse-grained. Glassy: very smooth, no visible grains. |
| Mineral Composition: Mostly antigorite, amesite, and lizardite. Sometimes: chrysotile (asbestos type), brucite, magnesite, chromite, magnetite, garnets. Talc often present from serpentine alteration. |
| Formation: Oceanic crust slices pushed onto continents during plate collision. Peridotite rock from oceanic plate bottom transforms to serpentinite due to reduced weight, lower temperature, and water circulation. Found in mountains once at continental edges. Also from peridotites crystallizing deep in crust from magma, later exposed by erosion and altering to serpentinite near surface. |
| Compare To: diabase, gabbro, slate, schist |
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Igneous Rocks
29.
| RHYOLITE (rye’-o-lite) |
|---|
| Rock Type: Igneous |
| Mineral Composition: Quartz, feldspars. Sometimes: biotite, diopside, hornblende, zircon. |
| Appearance: Usually light-colored: light gray, tan, reddish, greenish, brown. Fine-grained, often with scattered larger crystals. May have small gas bubble pockets. Sometimes shows flow lines or bands. |
| Formation: Volcanic rock. Rapid cooling of silica-rich magma or lava. Molten material often contains gas bubbles frozen into the rock. Pumice is a rhyolite type with many tiny gas bubbles. |
| Compare To: pumice, basalt |
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30.
| GRANITE (gran’-it) |
|---|
| Rock Type: Igneous |
| Mineral Composition: Quartz, feldspars (microcline, orthoclase, albite), biotite, muscovite. Sometimes: hornblende, augite, magnetite, zircon. |
| Appearance: Feldspars determine color: white to light gray, yellowish, or pink. Quartz usually smoky gray or white. Common black biotite specks, sometimes hornblende. Silvery to brownish muscovite common. Coarse to very coarse-grained. Random crystal arrangement (unlike layered gneiss). |
| Formation: Deep in Earth’s crust from cooling silica-rich magma. Slow cooling creates large crystals. |
| Compare To: gneiss diorite |
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31.
| DIORITE (die’-or-ite) |
|---|
| Rock Type: Igneous |
| Mineral Composition: Dark plagioclase, hornblende, pyroxene, sometimes a little quartz. May contain small amounts of light plagioclase feldspars. |
| Appearance: Looks like dark granite. Dark plagioclase and pyroxenes give darker color. Usually medium to dark gray. Unlike granite, little to no mica, and if present, it’s dark. Coarse-grained (larger than rice). |
| Formation: Deep in Earth’s crust from cooling magma, like granite. Magma lacks quartz and light minerals of granite, instead contains only dark minerals. |
| Compare To: granite, diabase |
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32.
| BASALT (buh-salt’) |
|---|
| Rock Type: Igneous |
| Appearance: Dark gray to black. Weathering can turn surface yellow or brown. Fine-grained. Crystals may or may not be visible with magnifier (often microscopic). Hard, tough, difficult to break. Sometimes contains gas bubbles (vesicular basalt). |
| Mineral Composition: Plagioclase feldspars, augite, hypersthene, olivine. |
| Formation: Volcanic rock. From iron and magnesium-rich, silica-poor magma (quartz-poor). Erupts from volcanoes or fissures as lava. Rapid cooling results in fine grains. |
| Compare To: rhyolite, diabase, gabbro |
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33.
| DIABASE (die’-uh-base) |
|---|
| Rock Type: Igneous |
| Appearance: Dark green to black, sometimes with scattered white crystals. Weathered surface often brown. Medium grain size (visible without magnifier, smaller than rice). Tough, hard rock. |
| Mineral Composition: Plagioclase feldspars, augite. Sometimes: hornblende, magnetite, olivine, glass. |
| Formation: From iron and magnesium-rich, silica-poor magma (quartz-poor). Magma forced into cracks or between rock layers near Earth’s surface. Same magma as basalt but cools slower, developing slightly larger crystals. |
| Compare To: basalt, gabbro, diorite, serpentinite |
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34.
| GABBRO (gab’-row) |
|---|
| Rock Type: Igneous |
| Appearance: Dark green to black. Weathered surface often brown. Large grain size (most grains larger than rice). |
| Mineral Composition: Plagioclase feldspars, augite, hypersthene, olivine. Sometimes: magnetite, chromite, titanite, ilmenite. |
| Formation: From iron and magnesium-rich, silica-poor magma (quartz-poor). Magma cools and crystallizes deep below Earth’s surface. Same magma as basalt and diabase, but slower cooling results in larger crystals. |
| Compare To: basalt, diabase, serpentinite |
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35.
| PUMICE (pum’-iss) |
|---|
| Rock Type: Igneous |
| Appearance: Very light gray to medium gray. Many gas bubbles surrounded by thin volcanic glass layer. Sponge-like appearance. Very light weight. Most pieces float on water. Flow lines or bands may be visible. |
| Mineral Composition: Glass; mineral grains are unusual. |
| Formation: Explosively ejected from volcanoes. From highly silicic, thick, sticky magma. Trapped gases in bubbles cause explosive eruption. Same magma type as rhyolite or granite. |
| Compare To: scoria, rhyolite |
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36.
| SCORIA (score’-ee-uh) |
|---|
| Rock Type: Igneous |
| Appearance: Usually black, dark gray, brown, or dark green. Glassy, smooth to rough, contains gas bubbles. Fewer, larger bubbles than pumice, moderately heavy (unlike pumice). |
| Mineral Composition: Mainly glass. |
| Formation: Usually from lava flow top, volcanic. From somewhat sticky lava. Top of flow cools quickly, limiting crystal formation. |
| Compare To: pumice, basalt |
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37.
| OBSIDIAN (obb-sid’-ee-an) |
|---|
| Rock Type: Igneous |
| Appearance: Glassy, usually black, sometimes grayish or greenish. May have white “snowflake” crystals (snowflake obsidian) or red swirls. Breaks and chips like glass, with scoop-shaped fracture (conchoidal fracture) and semicircular ridges. |
| Mineral Composition: Black glass. |
| Formation: Volcanic. Rapid cooling of silica-rich lava. Cools too fast for crystals to form. |
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Sedimentary Rocks
38.
| SANDSTONE (sand’-stone) |
|---|
| Rock Type: Sedimentary |
| Appearance: Often red to brown, light gray to nearly white. Sometimes yellow or green. Typically rounded grains of uniform size, usually medium-grained. Some show slight color variations in layering. |
| Mineral Composition: Quartz. Sometimes: feldspars, mica, glauconite (green sandstone), magnetite, garnet, rutile, ilmenite. |
| Formation: Quartz sand from weathering of rocks (granite, gneiss, other sandstones) deposited by rivers, waves, or wind (sandbars, beaches, dunes). Buried under sediment, compacted, and cemented by dissolved minerals in water. |
| Related Rocks: Arkose: Usually red or pink, may be gray. Angular grains. >25% feldspar with quartz. Medium to coarse-grained. Greywacke: Black or dark green. Coarse angular grains with fine grains. |
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39.
| SHALE (sh-ale) |
|---|
| Rock Type: Sedimentary |
| Appearance: Black, gray, red, brown, dark green, or blue. Fine-grained, particles usually not visible. Wet shale smells like wet mud. |
| Mineral Composition: Clay minerals. Sometimes quartz sand, pyrite, gypsum. |
| Formation: Clay sediments settle in quiet lakes, lagoons, bays, or offshore areas. Buried and compacted clays become shale. Iron oxides often help cement particles. |
| Compare To: slate schist |
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40.
| LIMESTONE (lime’-stone) |
|---|
| Rock Type: Sedimentary |
| Appearance: Usually white, gray, tan, or yellow. Impurities can make it red or black. Often contains fossils. Can be smooth, sugary, fine-grained, or medium-grained. Powdered rock usually fizzes in white vinegar. Unlike marble, not composed of visible crystals. |
| Mineral Composition: Mostly calcite. |
| Formation: Most limestone forms from chemical reaction in seawater, creating lime mud that settles to form limestone. Some from buried coral reefs. |
| Related Rocks: Dolostone (doe’-low-stone): Looks like limestone, but mineral is dolomite. Powdered dolostone does not fizz with white vinegar. Forms on ocean floor. |
| Compare To: marble |
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41.
| CONGLOMERATE (cun-glom’-er-at) |
|---|
| Rock Type: Sedimentary |
| Appearance: Mixture of sand and rounded pebbles of different sizes. Pebbles are key observation. |
| Mineral Composition: Mostly quartz. |
| Formation: Sand and pebbles collect along seashores, lake shores, or riverbanks. Compacted by overlying sediment and cemented by dissolved minerals in water. |
| Related Rocks: Breccia (brech’-ee-uh): Looks like conglomerate, but “pebbles” are jagged and blocky, not rounded. |
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42.
| BRECCIA (brech’-ee-uh) |
|---|
| Rock Type: Sedimentary |
| Appearance: Like conglomerate, but “pebbles” are jagged and blocky, not rounded. |
| Mineral Composition: “Cement” is mostly quartz, pebbles can be any rock type – often quartzite, granite, or tough rocks resistant to erosion. |
| Formation: Dry environments like deserts. Eroded rock pieces from mountains don’t get carried by streams, just pile up. Deep piles get compressed and cemented. |
| Compare To: Conglomerate |
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About the Rock Identification Key
This Rock Key is designed to help both children and adults identify common rocks found in everyday settings and during travels. It is freely available for non-commercial use.
Special thanks to Alan Plante for his valuable suggestions and editorial contributions that significantly improved this Rock Key.
This Rock Identification Key, in a flowchart format, has been successfully used with over 6000 elementary school children for over 20 years. This web version aims to be equally effective.
The Key focuses on common rock types, but some rocks may not be included. In such cases, the identification process should lead to a closely related rock type.
While copyrighted, The Rock Identification Key can be freely distributed for non-commercial use by individuals, educators, and organizations. However, it is not authorized for inclusion in any publication, print or electronic, that is sold or requires a fee.
Enjoy exploring the world of rocks and confidently name your discoveries!
© Donald. B. Peck, 2001
Don Peck, [email protected]
6 Indian Rock Road, Warren, New Jersey 07059
