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Ministry of Energy Mines and Responsible for Core Review

Common Rock Types



Sandstone consists of rounded sand grains cemented together; it is one of the most common sedimentary rocks. A sandstone may contain obvious layers (bedding) or lines at angles to the layers (cross-bedding), but in some cases the beds are too far apart to see in the one exposure, as is the case with conglomerate.
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Conglomerate is a cemented-together gravel that consists mostly of rounded rock fragments which are greater than 2 millimetres in size. The spaces between the grains are filled with varying amounts of fine-grained material or cement. Conglomerate, as with most sedimentary rocks, is laid down in layers, however, these layers are often so thick that they may not be visible in an isolated outcrop.
pebble conglomerate




Breccia is similar to conglomerate in that it consists of coarse-grained fragments surrounded by finer grained material. The fragments in a breccia, however, are sharp and angular, looking as if they were just broken, while those in conglomerates are rounded.
chert breccia



Volcanic Breccia

Volcanic breccia consists of a variety of coarse angular volcanic fragments in a matrix of finer grained tuff.
volcanic breccia
Marble is limestone or dolomite that has been recrystallized due to metamorphism. It is a light-coloured rock and consists of medium to coarse-grained interlocking calcite or dolomite crystals. A marble formed of calcite crystals will fizz with dilute acid, while a dolomite marble will fizz only if powdered, or if hot acid is used. Darker streaks are often present in marble as are calcite veins. Marble scratches easily with a knife.
Coarse-grained marble
marble marble
Marble Marble
Finer grained grey marble with impurities
Pegmatite is a very coarse-grained (more than 2 centimetres) intrusive igneous rock. They usually occur as veins or dykes within granite or syenite and contain similar minerals - quartz, orthoclase feldspar, plagioclase feldspar and mica. They may also contain much less common minerals such as tourmaline, beryl, etc.
Granitic pegmatite
(the bluish-green mineral is a variety of
feldspar called Amazonite)
A porphyry is an igneous rock that contains some large crystals floating in a fine-grained matrix. This is really a textural term and the rock is usually named for its overall composition, which may vary from felsite through andesite to basalt. However, this composition may not always be determinable without laboratory investigation, particulary for felsic to intermediate composition rocks. Consequently, the rock may be called a "quartz porphyry", "feldspar porphyry", etc., depending upon the nature of the phenocrysts (the large crystals) present.
hornblende-feldspar porphyry
Granodiorite is an intermediate coloured, medium to coarse-grained intrusive rock. It falls between granite and quartz diorite, containing more dark minerals than granite but less than quartz diorite. The light coloured minerals are quartz and feldspar; the feldspar is a mixture of orthoclase and plagioclase. The dark coloured crystals are hornblende and/or biotite. Granodiorite is a very common intrusive rock in British Columbia.
hornblende granodiorite
Hornblende granodiorite
granodiorite biotite granodiorite
Granodiorite Biotite granodiorite
Hornblende-biotite granodiorite
Hornblende-biotite granodiorite
Gabbro is a dark, medium to coarse-grained intrusive rock. It contains more than 50% dark crystals interlocking with light crystals. The dark minerals are mainly pyroxene with minor amounts of hornblende or olivine; the light minerals are usually; light grey plagioclase feldspars.
biotite gabbro
Biotite gabbro
pyroxene gabbro
Pyroxene gabbro
If there are no light crystals at all, the rock is an ultramafic
Ultramafic intrusive rocks consist of only dark-coloured minerals, mostly pyroxene and olivine. Rocks composed almost entirely of dark-coloured plagioclase (known as "anorthosites")are often associated with ultramafic rocks.
olivine pyroxenite wehrlite
Coarse olivine pyroxenite Weathered ulramafic rock
Note: distinctive brown surface colour
Dark coloured anorthosite

Ultramafic rocks are often altered to serpentinite.

Syenite is a light coloured coloured intrusive rock. It is similar to granite except that it contains very few or no quartz grains. It is made up predominantly of interlocking feldspar crystals with minor amounts of mica or hornblende. Syenite is rare in British Columbia.
porphyritic syenite syenite
Porphyritic syenite Syenite

If the crystals are very large (more than 2 centimetres), the rock can be called a pegmatite.

Granite is a pink or greyish coloured rock and represents the lightest coloured variety of intrusive rock. It is medium to coarse-grained and evenly granular. The grains are mainly white to pink orthoclase feldspar with lesser amounts of white to grey plagioclase feldspar and quartz; small amounts of dark biotite and/or hornblende are mixed in with these light mineral components. In some granites, where the feldspar is red in colour, the rock will appear darker. True granite is not common in British Columbia.
granite granite
Granite Granite
biotite granite
Biotite granite
Quartz diorite is a medium to dark grey, medium to coarse-grained intrusive rock. It consists mainly of plagioclase feldspar, some quartz, and abundant dark minerals (roughly a third). The dark crystals are usually hornblende and biotite, with minor pyroxene. Quartz diorite is common in British Columbia.
quartz diorite
Quartz diorite
quartz diorite
Quartz diorite

If there is no quartz the rock is a diorite

Diorite is a medium to dark grey, medium to coarse-grained intrusive rock. Light minerals are mainly plagioclase feldspar with no quartz. Dark minerals are abundant, though less than light crystals, and include pyroxene and hornblende.
hornblende diorite
Hornblende diorite
When quartz is present, the rock is a quartz diorite. When dark minerals are more abundant than light minerals the rock is a gabbro.
Quartzite is a metamorphosed quartz sandstone. It is a very hard, sugary textured rock that consists of interlocking quartz grains. The original quartz sand grains are enclosed in and bound together by quartz cement. This makes the rock hard; when hammered, it breaks across the grains and cement as opposed to around the grains as is the case in sandstones. Freshly broken faces have a glassy appearance. Quartzite is generally light in colour but impurities such as mica are common and any gradation from quartzite to schist may occur. Quartzite may retain some of the original bedding of the sandstone.
Schist (pronounced SH-ist) is a metamorphic rock which is intermediate between slate and gneiss. The rock has layers consisting of aligned mica or mica-like minerals which give the rock a glistening appearance. It splits readily along these layers and the broken faces usually consist of wavy or lens-shaped slabs or flakes rather than thin sheets.
Gneiss (pronounced NICE) is a medium to coarse-grained, banded, granular metamorphic rock. Disinct colour bands or streaks are produced by the alternation of layers of light (e.g. quartz and feldspar) and dark-coloured (e.g. biotite and hornblende) minerals. The layers do not split readily and when broken are not smooth. Gneiss may form from diorite, granite, shale, sandstone, schist or other rocks.
granitic gneiss
Granitic gneiss
(quartz, orthoclase, biotite)
granodiorite gneiss gneiss
Granodiorite gneiss Gneiss
Obsidian is a natural volcanic glass. It is generally dark in colour with a bright glassy lustre, though weathered surface can show a dull white patina. It breaks with curved striated faces, like glass, known as conchoidal fracture. Obsidian is not common in British Columbia.
Chert is a hard, compact, dense, brittle rock which generally varies in colour from white, through light grey to dark grey. The appearance is waxy and smooth; quartz, with which chert might be confused, is glassy. When struck with a hammer, chert breaks with a conchoidal fracture, like broken glass, or shatters into splinters. These properties result because chert consists of very fine-grained silica. Chert occurs as nodules or irregular bands in limestone, or as thin beds or ribbon-shaped layers separated by thin films of shale.
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Tuff is lithified volcanic ash. It contains a variety of small angular fragments that were blown out of the volcano into the air. As the fragments fall, the larger and heavier ones fall faster and the tuff may show layering like a sedimentary rock.
tuff.gif (421063 bytes)
Felsite is a general term for all light-coloured volcanic rocks. The most common variety, which is the fine-grained or volcanic equivalent of granite, is called rhyolite. Felsites are generally granular, rough to the touch, and in some cases show a series of more or less parallel bands of different colours called flow bands. These may be wavy and swirly and can therefore be distinguished from sedimentary bedding or metamorphic foliation. Felsites are not common in British Columbia but can be important indicators for exploration of some metallic mineral deposits.
Andesite is intermediate in composition and colour between felsite and basalt. It is the fine-grained volcanic equivalent of diorite and is generally a medium to dark grey rock consisting of mainly plagioclase feldspar and abundant dark minerals. Colours range from grey and grey-green to red or pink. Andesites are generally porphyritic, that is, they contain some large crystals floating in a fine-grained matrix. In British Columbia, andesites are more common than felsites, but less common than basalts.
Basalt is a fine-grained, dark-coloured, dense rock which has a dull, granular appearance. Basalt, the volcanic equivalent of gabbro, is often porphyritic. Commonly basalt is vesicular, that is, it has holes (vesicles) which were once gas bubbles; later, these holes often fill with mineral matter (amygdules). Basalt which formed by underwater eruptions charateristically develop rounded pillow-like shapes. Basalt is a very common rock in British Columbia.
vesicular basalt
Limestone is a white, grey or black, fine-grained rock which fizzes vigorously when a drop of dilute hydrochloric acid is put on it. It commonly contains fossils such as shells or other animal remains, and in some cases the fossils may make up most of the rock. When limestone is hit with a hammer it may give off a sulphurous smell. Calcite is the dominant constituent in limestones and it is the material which causes the rock to effervece with acid.
fossiliferous limestone
Fossiliferous limestone
limestone crinoidal limestone
Fine-grained limestone Crinoidal limestone
black limestone
Black limestone with white calcite veins
Note: Calcite is also a common material in other sedimentary rocks, where it can occur in fossils, in vein and fracture coatings, or as a cement. For this reason one must be careful when testing for a reaction to acid, and put the drop of acid on a fresh rock surface which does not contain fossils or veins.
Dolomite looks very much like limestone except that it is generally tan in colour and fizzes weakly if at all when a drop of cold dilute hydrochloric acid is put on it. If dolomite is ground to a powder or hot acid is applied, it will fizz strongly. Limestone and dolomite also weather differently: weathered dolomite often becomes brown or reddish and rough and finely angular; limestone, on the other hand, weathers smooth and light grey or white.
Shale has the apearance of earthy, hard, mud or clay, which is not surprising since shale is a compacted version of these materials. Shales vary in colour but are generally grey to black or brown. Shale is a soft rock and can easily be scratched with a knife. When hit with a hammer, a mark will remain and the rock breaks into chunks as oppsed to sheets (as would be the case with metamorphosed shale - slate). The beds in shale are usually thin, flat and of uniform thickness.
Slate is the metamorphic product of shale. Slate is a fine-grained brittle rock which splits readily into thin smooth-faced layers or sheets. Slates are generally dark grey, green or black in colour, but may also be red or brown. Individual grains cannot be distinguished with the naked eye. The term argillite refers to rocks with characteristics intermediate between shale and slate; they are harder than shale but do not break into sheets like slate.
Serpentinite is an oily looking green to black fine-grained rock that can be scratched easily with a knife. It forms by the metamorphism of ultramafic rocks. In many places serpentinite is highly sheared and breaks into scaly fragments with smooth shiny faces and a slippery feel. At thin edges the fragments tend to be translucent. Serpentinite is composed of the mineral serpentine.
Serpentinite with veins of talc
serpentinite with asbestos vein
Serpentinite with asbestos vein
Soapstone is a rock composed of the mineral talc and can be mistaken for serpentinite. Soapstone, however, is usually light green to grey in colour and soft enough to be scratched with the fingernail.