preproom.orgThe online science prep room
You are here: Info Library > Earth and Space > Rock Types
Info Library

Earth and Space > Rock Types

Below are details of the most commonly used *rocks in schools (click on each picture for a larger view with ruler for scale).

Basalt

Fig 1: Basalt
Fig 1:
Basalt
Type
Igneous (extrusive volcanic).
Composition
Feldspar, pyroxene, amphibole, olivine.
Uses
Crushed and used as concrete aggregate, fibres used in glass wool and some roofing felts.
Identification
Red-brown to black, usually fine grained. May have small visible holes where gas escaped while the lava cooled, Unweathered basalt is black or dark grey.
Notes
Basalt is solidified lava.

Slate

Fig 2: Slate
Fig 2:
Slate
Type
Metamorphic.
Composition
Quartz and muscovite, often also with biotite, chlorite, hematite, and pyrite.
Uses
Roofing, blackboards, whetstones.
Identification
Dark grey to black, maybe almost purple. Very fine grains (smooth to the touch), harder than shale, layers may be visible.
Notes
Can be spilt into thin sheets.

Shale

Fig 3: Shale
Fig 3:
Shale
Type
Sedimentary.
Composition
Clay grains.
Uses
Some brick manufacture.
Identification
Breaks easily. Will leave a muddy streak if dipped in water and drawn along a surface. Finely grained, breaks with an irregular curving fracture. Dull brown-red.
Notes
The process in the rock cycle which forms shale is called 'compaction'.

Marble

Fig 4: Marble
Fig 4:
Marble
Type
Metamorphic.
Composition
Very pure, recrystallized calcite through metamorphism of limestone or dolostone.
Uses
Worktops, tombstones and Sculptures.
Identification
White or light coloured, dense and hard. Small calcite grains may sparkle in the light.
Notes
Colorless or light-colored marbles are a very pure source of calcium carbonate.

Sandstone

Fig 5: Sandstone
Fig 5:
Sandstone
Type
Sedimentary.
Composition
Comprised of sand sized rock grains, qquartz and/or feldspar.
Uses
Flagstones, building material.
Identification
Visible grains of sand, beige to orange in colour, feels like sandpaper to the touch.
Notes
Formed from cemented grains that may be fragments of a pre-existing rock.

Limestone

Fig 6: Limestone
Fig 6:
Limestone
Type
Sedimentary.
Composition
Mostly calcite, often contains variable amounts of silica in the form of flint, as well as varying amounts of clay, silt and sand.
Uses
Used in the production of glass, cement and mortar and used as a soil conditioner to neutralize acidic soil conditions among other uses.
Identification
White-grey with a chalky texture. Some samples are harder than others. May be crystalline, clastic, granular, or massive, depending on the method of formation.
Notes
Limestone and marble are very reactive to acid solutions, making acid rain a significant problem.

Schist

Fig 7: Schist
Fig 7:
Schist
Type
Metamorphic.
Composition
Schist is foliated, meaning the individual mineral grains split off easily into flakes or slabs.
Uses
Was used in the 20s and 30s mainly in building work in the USA.
Identification
Usually brownish-grey with specks of mica, chlorite, talc, hornblende, graphite, quartz and others.
Notes
By definition, schist contains more than 50% platy and elongated minerals, often finely interleaved with quartz and feldspar.

Obsidian

Fig 8: Obsidian
Fig 8:
Obsidian
Type
Igneous.
Composition
A type of naturally-occurring glass formed as an extrusive igneous rock.
Uses
Has been used to make arrow heads and mirrors. Fine obsidian blades can be used in surgery.
Identification
Usually black, hard shiny glasslike with no crystals. May have lightly coloured 'snowflake' patches. Can be fractured to produce sharp edges.

Quartzite

Fig 9: Quartzite
Fig 9:
Quartzite
Type
Metamorphic.
Composition
Recrystallized quartz grains.
Uses
Glass and ceramic manufacture, railway ballast.
Identification
Light grey to white, hard, medium grained. Some samples may appear pink to red due to iron oxide which may be present.
Notes
Very resistant to chemical weathering.

Gneiss

Fig 10: Gneiss
Fig 10:
Gneiss
Type
Metamorphic.
Composition
Feldspar, quartz, mica.
Uses
Ornamental uses.
Identification
Banded with alternating layers of dark and light minerals Gneiss resembles schist, except that the minerals are arranged into bands.
Notes
The name 'gneiss' is from an old Saxon mining term that seems to have meant decayed, rotten, or worthless material.

Malachite

Fig 11: Malachite
Fig 11:
Malachite
Type
Mineral.
Composition
Malachite is a carbonate mineral; copper(II) carbonate hydroxide.
Uses
Ornamental, jewellery, copper ore.
Identification
Green in colour may have a 'bubbly' appearance on the outside surface. May be banded light and dark green.
Notes
Named after the Greek word for 'mallow', a green herb.

Hematite

Fig 12: Hematite
Fig 12:
Hematite
Type
Mineral.
Composition
Hematite, is the mineral form of Iron(III) oxide.
Uses
Iron ore.
Identification
Colored black to steel or silver-grey, brown to reddish brown, or red.
Notes
Can also be spelled 'Haematite'.

Mudstone

Fig 13: Mudstone
Fig 13:
Mudstone
Type
Sedimentary.
Composition
Formerly clay or mud.
Identification
Grey to brown colour, looks like hardened clay. Grain size is up to 0.0625mm.
Notes
Mud rocks comprise some 65% of all sedimentary rocks.

Chalk

Fig 14: Chalk
Fig 14:
Chalk
Type
Sedimentary.
Composition
A form of limestone composed of calcite.
Uses
Blackboard chalk, antacids.
Identification
White in colour, soft, fizzes when mixed with HCl.
Notes
It forms from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores.

Gabbro

Fig 15: Gabbro
Fig 15:
Gabbro
Type
Igneous.
Composition
Feldspar, pyroxene, amphibole, olivine.
Uses
Often contains valuable chromium, cobalt, gold, silver, platinum or nickel.
Identification
WDark grey-black, shiny surfaces of feldspar may be visible. May appear similar to granite. Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater.

Conglomerate

Fig 16: Conglomerate
Fig 16:
Conglomerate
Type
Sedimentary.
Composition
Fragments of other rocks and minerals cemented by silica, calcite, or iron oxide.
Uses
Used in construction and building.
Identification
Grey rock with visible fragments of other rocks and minerals.

Dolomite

Fig 17: Dolomite
Fig 17:
Dolomite
Type
Sedimentary and Mineral.
Composition
Composed of calcium magnesium carbonate.
Uses
Ornamental stone.
Identification
White, grey to pink, commonly curved crystals. Manganese and iron may give dolomite a pinky or yellow tinge.
Notes
Dolomite rock may be referred to as dolostone.

Granite

Fig 18: Granite
Fig 18:
Granite
Type
Igneous.
Composition
Mica, feldspar, quartz, hornblend.
Uses
Worktops, gravestones.
Identification
Visible crystals of pink feldspar, white or grey quartz, and black mica, hard and tough.
Notes
Granite contains around 10 to 20 parts per million of uranium.

Rhyolite

Fig 19: Rhyolite
Fig 19:
Rhyolite
Type
Igneous.
Composition
Feldspar, quartz, mica, hornblend.
Identification
Very fine grained, pinkish-grey, may have dark streaks.

*All rock images (except Gneiss, Dolimite and Chalk) are © Alan Scott, Ph.D. University of Wisconsin-Stout.

Related Links

Rock Identification (table)

Rock Identification Key