Study Guides | earth | Identifying Rocks

Study Guide: Identifying Rocks

To identify a rock, think like a geologist and examine its physical characteristics for clues.
  • Three steps to identify a rock:
    1. Decide if it is igneous, sedimentary or metamorphic.
    2. Study it’s physical characteristics, such as grain size, hardness, luster, transparency, density, crystal shape, break pattern, and color.
    3. Look it up in a chart, such as the ones on this page.
  • Be careful with color. It is not always a reliable way to identify rocks.
  • The Mohs Test, invented by Friedrich Mohs, is used to compare the hardness of rocks.
  • The softest rock is Talc (1 on the Mohs test), and Diamond is the hardest rock (10 on the Mohs test).

Rock Identification in 3 Steps

1: Decide whether the rock is igneous, sedimentary or metamorphic

Igneous rocks such as granite or lava are tough, frozen melts with little texture or layering. Rocks like these contain mostly black, white and/or gray minerals.

Sedimentary rocks such as limestone or shale are hardened sediment with sandy or clay-like layers (strata). They are usually brown to gray in color and may have fossils and water or wind marks.

Metamorphic rocks such as marble are tough, with straight or curved layers (foliation) of light and dark minerals. They come in various colors and often contain glittery mica.

2: Check the grain size and hardness

Grain Size: Coarse grains are visible to the naked eye, and the minerals can usually be identified without using a magnifying glass. Fine grains are smaller and usually cannot be identified without using a magnifying glass.

Hardness: This is measured with the Mohs scale and refers to the minerals contained within the rock. Hard rock scratches glass and steel, usually signifying the minerals quartz or feldspar, which has a Mohs hardness of 6 or higher. Soft rock does not scratch steel but will scratch fingernails (Mohs scale of 3 to 5.5). Very soft rock won’t even scratch fingernails (Mohs scale of 1 to 2).

3: Try to identify the rock

Here are lists of many common rocks with some observable characteristics.

Igneous Rock Identification

Grain Size Usual Color Other Composition Rock Type
fine dark glassy appearance lava glass Obsidian
fine light many small bubbles lava froth from sticky lava Pumice
fine dark many large bubbles lava froth from fluid lava Scoria
fine or mixed light contains quartz high-silica lava Felsite
fine or mixed medium between felsite and basalt medium-silica lava Andesite
fine or mixed dark has no quartz low-silica lava Basalt
mixed any color large grains in fine-grained matrix large grains of feldspar, quartz, pyroxene or olivine Porphyry
coarse light wide range of color and grain size feldspar and quartz with minor mica, amphibole or pyroxene Granite
coarse light like granite but without quartz feldspar with minor mica, amphibole or pyroxene Syenite
coarse light to medium little or no alkali feldspar plagioclase and quartz with dark minerals Tonalite
coarse medium to dark little or no quartz low-calcium plagioclase and dark minerals Diorite
coarse medium to dark no quartz; may have olivine high-calcium plagioclase and dark minerals Gabbro
coarse dark dense; always has olivine olivine with amphibole and/or pyroxene Peridotite
coarse dark dense mostly pyroxene with olivine and amphibole Pyroxenite
coarse green dense at least 90 percent olivine Dunite
very coarse any color usually in small intrusive bodies typically granitic Pegmatite

Sedimentary Rock Identification

Hardness Grain Size Composition Other Rock Type
hard coarse clean quartz white to brown Sandstone
hard coarse quartz and feldspar usually very coarse Arkose
hard or soft mixed mixed sediment with rock grains and clay gray or dark and “dirty” Wacke/Graywacke
hard or soft mixed mixed rocks and sediment round rocks in finer sediment matrix Conglomerate
hard or soft mixed mixed rocks and sediment sharp pieces in finer sediment matrix Breccia
hard fine very fine sand; no clay feels gritty on teeth Siltstone
hard fine chalcedony no fizzing with acid Chert
soft fine clay minerals splits in layers Shale
soft fine carbon black; burns with tarry smoke Coal
soft fine calcite fizzes with acid Limestone
soft coarse or fine dolomite no fizzing with acid unless powdered Dolomite rock
soft coarse fossil shells mostly pieces Coquina
very soft coarse halite salt taste Halite or Rock Salt
very soft coarse gypsum white, tan or pink Rock Gypsum

Metamorphic Rock Identification

Foliation Grain Size Usual Color Other Rock Type
foliated fine light very soft; greasy feel Soapstone
foliated fine dark soft; strong cleavage Slate
nonfoliated fine dark soft; massive structure Argillite
foliated fine dark shiny; crinkly foliation Phyllite
foliated coarse mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite
foliated coarse mixed dark and light wrinkled foliation; often has large crystals Schist
foliated coarse mixed banded Gneiss
foliated coarse mixed distorted “melted” layers Migmatite
foliated coarse dark mostly hornblende Amphibolite
nonfoliated fine greenish soft; shiny, mottled surface Serpentinite
nonfoliated fine or coarse dark dull and opaque colors, found near intrusions Hornfels
nonfoliated coarse red and green dense; garnet and pyroxene Eclogite
nonfoliated coarse light soft; calcite or dolomite by the acid test Marble
nonfoliated coarse light quartz (no fizzing with acid) Quartzite

Mineral and Gems Identification

Color

Minerals are found in a variety of colors due to the chemicals in them. A fresh surface is usually needed to see the true color as weathering may hide it.

Luster

The amount of light reflected from a mineral’s surface. Luster may be described as glassy, metallic, shiny, dull, waxy, satiny, or greasy.

Streak Color

The color of a mark a mineral makes on a white tile. (The color of the mineral’s powder.) The streak may not be the same as the mineral’s color.

Hardness

Hardness is the ability of a mineral to resist being scratched. Any mineral can scratch other minerals that are softer than itself, but can be scratched by other mineral that are harder.

Hardness plays a major role in identifying minerals. It can make the process simpler by quickly narrowing down the search.

Scratch tests are often done with common objects.

Friedrich Mohs

Friedrich Mohs, a German mineralogist, developed the scale in 1812. He selected ten minerals of distinctly different hardness that ranged from a very soft mineral (Talc) to a very hard mineral (Diamond). With the exception of Diamond, the minerals are all relatively common and easy or inexpensive to obtain.

Note on spelling: Friedrich’s last name is Mohs. Therefore we don’t call the test Moh’s or Mohs'.

Mohs Hardness Scale

One of the most important tests for identifying minerals is the Mohs Hardness Test. This test compares the resistance of a mineral to being scratched by ten reference minerals known as the Mohs Hardness Scale. The test is useful because most minerals of a given type are very close to the same hardness. This makes hardness a reliable property for testing most minerals.

Mineral Hardness
Talc 1
Gypsum 2
Calcite 3
Fluorite 4
Apatite 5
Orthoclase 6
Quartz 7
Topaz 8
Corundum 9
Diamond 10

Crystal Shape

The atoms of most minerals are arranged in orderly, repetitive patterns called crystals. Crystals that grow in an open space can be almost perfectly formed. Crystals that grown in a tight space are often incompletely formed.

Break Patterns

  • Cleavage: A type of break in a mineral that has long flat surfaces.
  • Fracture: A type of break in a mineral that breaks unevenly and does not leave flat surfaces when they break.

Other Characteristics

  • Odor: Certain minerals have very distinctive smells, such as Sulphur which smells like rotten eggs.
  • Taste: Salt can be easily identified by taste.
  • Texture: Rough or smooth.
  • Transparency:
  • Density: Density refers rto how tightly packed the matter in an the mineral is. The denser the mineral, the heavier it will feel. Some Pumice rocks even float on water. $$Density = \dfrac{\textit{Mass}}{\textit{Volume}}$$.

Pumice floating on water

More Information

Minerals By Name
Mineral by Characteristics
Mineral by Properties
Identifying Minerals – Earth Rocks! (16:04)
How to identify a Mineral (8:40)
Source: https://class.ronliskey.com/study/geology/9_identifying_rocks/