What is your rock’s name?
Lots of people love rocks but don’t know the rock names. Got a rock you want to identify? Maybe you found it in a rock outcrop. Or did you pick it up after waves, currents, or glaciers dropped it off for you to find? You have a fascinating rock and no geologist in sight. What to do?
How do geologists identify rocks?
Since rocks are made of minerals, they use physical and chemical tests learned in mineralogy classes to determine the properties of the minerals in the rock. Then, using (or remembering) charts that categorize minerals by properties, they choose minerals that share properties revealed by the test results. That done, they use their knowledge of which rocks contain those minerals, and select the rock names which most closely match the correct minerals. Incorporating any additional features and information learned in their petrology classes, they make a final selection and interpret the origin and history of the rock.
Too complicated? You just want to know the name of your rock? Here’s a less sophisticated route!
Don’t worry about mineral names, just limit your tests to properties that the minerals contribute to the rock. You’ll have a faster and simpler, and less detailed route to the name for your treasure.
Enter Argeecee’s RAPID ID ROCK KEY. It allows for quick, accurate identification by observing rock appearance and using tests with high tech tools like glass, nails, fingernails, magnifying loupes, drops of HCL, etc. A detailed explanation of the tests and how to do them follows the RAPID ID ROCK KEY below.
Argeecee’s RAPID ID ROCK KEY
Examine a freshly broken face of the rock to see mineral crystals and grains. Weathered faces with altered minerals may not be representative of entire rock. Once the rock’s name is determined, go the end for a detailed description of rock types and names.
- will scratch glass – choose 3 or 4
- won’t scratch glass – choose 33, 34,or 52
- grains intergrown – choose 13, 14, or 15
- grains not intergrown – choose 5, 6, or 7
- coarse grains – choose 8 or 9
- medium grains – choose 10, 11, or 12
- fine grains – Siltstone – clastic sedimentary rock
- angular grains – Breccia – clastic sedimentary rock (see 55)
- rounded grains – Conglomerate – clastic sedimentary rock (see 56)
- flat, shiny pink or white grains in light colored rock – Arkose Sandstone – clastic sedimentary rock
- rounded, shiny, clear grains in light colored rock – Quartz Sandstone – clastic sedimentary rock
- rounded, shiny, clear grains in gray to black colored rock – Graywacke Sandstone – clastic sedimentary rock
- coarse to medium or fine grains, random grain arrangement – choose 16 or 17
- coarse to medium grains with foliation – Schist – foliated metamorphic rock
- coarse to medium grains with foliation and color bands – Gneiss – foliated & banded metamorphic rock
- more than one mineral (spotted appearance) – choose 18, 19, 20, 21, or 22
- one kind of mineral in rock (consistent color throughout) – choose 23, 24, or 25
- shiny, smoky clear rounded quartz grains, pink or white flat feldspar grains, and some dark grains in light colored rock – Granite – plutonic igneous rock
- pink or white flat grains and some dark grains in light colored rock – Syenite – plutonic igneous rock
- shiny, gray flat grains & shiny, dark flat grains in a light gray to dark gray colored rock – Diorite – plutonic igneous rock
- shiny, gray flat grains & dull, dark flat grains in a medium to dark colored rock – Gabbro – plutonic igneous rock
- shiny, dark gray flat grains, shiny green rounded grains & dull, dark flat grains in a dark colored rock – Peridotite – plutonic igneous rock
- very fine grains and smooth – choose 29, 30, 31, or 32
- fine grains and slightly rough – choose 26, 27, or 28
- coarse to medium grains – Metaquartzite – non-foliated metamorphic rock
- dark red to black colored – Basalt – volcanic igneous rock
- gray colored – Andesite – volcanic igneous rock
- pink, white to light gray color – Rhyolite – volcanic igneous rock
- dark gray to black color – Flint – chemical sedimentary rock
- red, yellow, or green color – Jasper – chemical sedimentary rock
- light gray to white color – Chert – chemical sedimentary rock
- curved color bands – Agate – chemical sedimentary rock
- bubbles in HCL – choose 35 or 36
- won’t bubble in HCL – choose 43, 44, 45, 46, 47, 48, or 52
- bubbles fast – choose 37, 38, 39, 40, 41, or 42
- bubbles slowly – Dolostone – chemical sedimentary rock
- no fossils, very coarse intergrown grains – Marble – non-foliated metamorphic rock
- fossils separated by other material – Fossiliferous Limestone – chemical sedimentary rock
- fossils loosely cemented – Coquina – chemical sedimentary rock
- coarse to medium grains – Crystalline Limestone – chemical sedimentary rock
- fine grains, very soft –Crystalline Limestone – chemical sedimentary rock
- very fine grains, smooth – Microcrystalline Limestone – chemical sedimentary rock
- softer than a fingernail – Rock Gypsum – chemical sedimentary rock
- tastes salty – Rock Salt – chemical sedimentary rock
- foliated or layered – choose 49, 50, or 51
- porous – choose 57 or 58
- glassy – Obsidian – volcanic igneous rock
- the powder that is form when scratching rock bubbles slowly in HCL – go to 36
- very fine grains, flat, dull – Shale – clastic sedimentary rock
- very fine to fine grains, flat, shiny – Slate – foliated metamorphic rock
- fine grains, bumpy, shiny – Phyllite – foliated metamorphic rock
- slightly softer than a nail – contains bits of glass – go to 54
- coarse – go to 18
- fine grains – Tuff – fragmental volcanic rock
- angular volcanic grains – Volcanic Breccia – fragmental igneous rock
- rounded volcanic grains – Agglomerate – fragmental igneous rock
- slightly porous, dark, and opaque – Scoria – volcanic igneous rock.
- porous, light colored, translucent, floats – Pumice – volcanic igneous rock.
ROCK ID TESTS
Hardness of minerals in rocks is measured by scratching specimen with minerals of known hardness from 1 to 10 with ten minerals utilized by Frederich Mohs. Scratching resistance is due to strength of chemical bonding. Alternatively, we can use:
- a glass plate or bottle – hardness 5.5
- a nail – hardness 5.0 (don’t use hardened steel)
- a penny – hardness 3.0
- a fingernail – 2.5.
You may also want bandages if you choose to ignore the proper procedure shown below on the left:
- Test diverse areas on the rock for an overall average.
- Start testing with glass, the hardest instrument. Place glass on table or other stable surface:
Be safe! If you try to scratch the rock by holding the glass in your hand, you may be cut when it splinters!
- Rub your rock on the glass. If the rock scratches glass, it is harder than 5.5. You may want to test the scratch with a fingernail. Can you can feel the groove? Rocks slightly harder than 5.5 leave shallow grooves, harder ones leave deeper grooves.
- If the rock won’t scratch glass it is softer than 5.5. It may even leave a streak of powder which can be wiped off, leaving no groove.
- It is softer than 5 if you can scratch it with a nail.
- If the nail can’t scratch it, hardness is 5 to 5.5.
- If the nail scratches it, you can also try the penny. Pennies scratch rocks less than 3.0. If the penny won’t scratch the rock and the nail will, hardness is 3.0 to 5.0.
- Your fingernail is next. Fingernails scratch rocks less than 2.5, if the fingernail won’t scratch the rock but the penny will, hardness is 2.5 to 3.0.
You can use these test results as one of the keys to unlock the rock name when you dive into Argeecee’s RAPID ID ROCK KEY above:
Intergrown Rocks – Observe how grains in the rock fit together. Intergrown grains fit together like puzzle pieces. Each is a crystal which grew until meeting a neighboring crystal. Non-intergrown or Fragmental Rocks have grains which do not fit together very well. Some are tightly packed, others are held together by cementing material.Grain Size – Without use of a magnifying glass or microscope, fine (silt) and very fine (clay) grains can’t be seen very well. Very fine rocks feel smooth when touched, fine grain rocks feel rough.
If you are into chewing rocks, fine grains feel gritty between your teeth, very fine grains are silky smooth. Medium (sand) or coarse (gravel) grains can be seen without a magnifying glass. Grains larger than 2 mm (~0.1 in.) are coarse gravel (see below).
Grain Color – Geologists classify brown, black, and medium to dark green rocks as dark colored. They regard white, tan, gray, red, and light green as light colors.
Grain Roundness – Grains with curved, smooth corners and edges are rounded. Those with pointed, sharp corners and jagged edges are angular. Use your 10X hand lens to observe roundness of medium, fine, and very fine grains.
Grain Parallelism –Foliated rocks exhibit platy (flat) or elongated grains that parallel one another. Foliation results from minerals responding to incredible pressures of mountain building. When squeezed, atoms and ions in minerals move and recrystallize in structures that efficiently resist or accept pressure.
Some foliated rocks have grains of similar color which are arranged in parallel layers of differing color, those I shown below are foliated and color banded.
FOLIATED & COLOR BANDED
HCL Reaction – Rapid reactions between rock and drops of dilute HCL can result in vigorous bubbling of carbonate minerals like calcite as carbon dioxide is generated. Slower reactions with some carbonates with stronger bonding such as dolomite form much smaller bubbles. It may require scratching the rock and placing the drops on the resulting powder in order to see the bubbles. Hot vinegar approximates reactions with room temperature HCL.
Geologists classify rocks into different groups based on how they are formed. You don’t necessarily need to know this to come up with a rock name, but it is pretty interesting to understand the fascinating processes that produce rocks.
Volcanic/Plutonic Igneous Rocks – Igneous is Latin for “Fire-formed” rocks, these rocks are formed by melting of preexisting rock. Volcanic rocks cool and crystallize rapidly at Earth’s surface from hot liquid lava; Plutonic rocks cool and crystallize slowly deep within the Earth from hot liquid magma.
Glassy Volcanic Igneous Rocks – Lava may cool so fast that ions don’t have enough time to migrate into regular arrangements so a crystalline solid doesn’t form. Instead, the rock is an amorphous solid and may be smooth and shiny like glass, or rough and porous if gas bubbles create holes in the cooling lava.
Metamorphic Rocks – Rocks heated and/or squeezed intensely may recrystallize without melting to form metamorphic rocks. Foliated metamorphic rocks have parallel platy or elongated minerals which non-foliated rocks lack.
Sedimentary Rocks – Clay, silt, sand or gravel grains break loose from preexisting rock and are deposited as sediment. If they are packed or cemented together they form clastic sedimentary rocks. Crystals precipitating from water solutions and deposited as sediment may recrystallize to form intergrown grains in chemical sedimentary rocks.
Fossil Rocks – Living organisms leave footprints or traces on or within Earth materials. After death, rock forming processes may preserve or copy parts of their bodies via chemical reactions. Such evidence of preexisting life is called a fossil and, when altered by chemical processes, are chemical sedimentary rocks.
ROCK NAMES (With typical included minerals)
So, you have determined your rock name from Argeecee’s RAPID ID ROCK KEY, if you want to know what minerals are probably within it, check out the following:
Breccia – angular rock, chert and / or quartz fragments cemented together
Conglomerate – rounded rock, chert and/or quartz fragments cemented together
Arkose – feldspar rich sandstone
Quartz Sandstone – quartz rich sandstone
Graywacke – quartz sandstone with clay and rock fragments
Siltstone – fine grained quartz and clay cemented together
Shale and Claystone – very fine grained clay and quartz packed together
Fossiliferous Limestone – fossil-rich rock with matrix of calcite between fossils
Coquina Limestone – fossils cemented by calcite; very little matrix
Crystalline Limestone – intergrown coarse/medium calcite crystals
Micrite Limestone – intergrown crystals of microscopic calcite
Dolostone – intergrown crystals of dolomite
Rock Gypsum – intergrown crystals of gypsum
Rock Salt – intergrown crystals of halite
Flint, Jasper, Chert, Agate – intergrown crystals of microscopic quartz colored by dispersed grains of iron oxide (hematite, limonite or magnetite) or trace elements
IGNEOUS – PLUTONIC
Granite – medium to coarse grain quartz, pink or white orthoclase and white to light gray sodium plagioclase feldspar, muscovite/biotite mica, hornblende amphibole, ++
Syenite – same as granite without smoky quartz, ++
Diorite – medium to coarse grain sodium plagioclase, amphibole (hornblende), ++
Gabbro – medium to coarse grain calcium plagioclase, augite pyroxene
Peridotite – medium to coarse grain calcium plagioclase, augite pyroxene, olivine
IGNEOUS – VOLCANIC
Rhyolite – same contents as granite, fine grained
Andesite – same contents as diorite, fine grained
Basalt – same contents as gabbro, fine grained
IGNEOUS – FRAGMENTAL
Tuff – rock formed from packed volcanic ash
Agglomerate – rock formed from packed volcanic ash and rounded volcanic fragments & bombs
Volcanic Breccia – rock formed from packed volcanic ash and angular volcanic fragments & bombs
IGNEOUS – GLASSY (amorphous solid)
Scoria – porous rock from frozen dark porous lava
Pumice – porous rock from frozen light colored air-borne lava drops
Obsidian – black or red volcanic glass
Note: Igneous rocks with two different grain sizes are named after the smallest grains (ground-mass). If the larger grains (phenocrysts) make up most of the rock the rock name is prefixed with the term “porphyritic” as in “porphyritic basalt”. If the ground-mass is more abundant than the phenocrysts, the term “porphyry” is used as a suffix as in “basalt porphyry”. “Amygdaloidal” is used as a prefix if the larger crystals are minerals which fill preexisting holes (vugs) formed by escaping volcanic gas.
METAMORPHIC – FOLIATED
Slate – very fine parallel grains of mica
Phyllite – fine parallel grains of mica
Schist – medium to coarse parallel grains of mica with quartz, feldspar, garnet, amphibole, +++
Gneiss – color banded coarse parallel grains of feldspar, quartz, amphibole, pyroxene, +++
METAMORPHIC – NON-FOLIATED
Marble – coarse intergrown grains of dolomite or calcite
Metaquartzite – coarse intergrown grains of quartz
Note: The prefix “porphyroblastic” is used to refer to metamorphic rock with two grain sizes.