ROCK CLASSIFICATION
NAME
COLOUR
GRAIN
SIZE/TEXTURE
MINERALS
STRUCTURE/
OTHERS
IGNEOUS ROCKS
ACID
Granite
Grey/pink speckled
Coarse, granular,
porphyritic
Microgranite
Grey/pink/yellow
Medium granular,
porphyritic
Rhyolite
Grey/pink/brown
Fine granular,
porphyritic
Obsidian
Shiny black, brown
or grey
None. glassy
INTERMEDIATE
Andesite
Grey, purple,
brown or green.
Fine granular,
porphyritic
BASIC
Gabbro
Basalt
Grey, black,
greenish
Black, dark grey,
green - mottled
Black, dark grey
Peridotite
Green-black
Coarse granular,
ophitic
Medium granular,
ophitic
Fine granular,
usually dense.
Porphyritic
Medium-coarse
granular
Mudstone/shale
Black, grey, brown,
red, green
Fine – minerals too
fine to recognise
Clay minerals +
quartz, feldspar
mica
Siltsone
Grey, black, brown,
yellow
Fine. Minerals can
just be
distinguished.
Even texture
Quartz, feldspar,
mica
Dolerite
Quartz, feldspar,
(or.+ pl.), mica,
(hornblende)
Quartz, feldspar
(or.+ pl.)mica,
(hornblende)
Quartz, feldspar
(or+pl), mica
(hornblende)
A glass, but with
rare phenocrysts of
quartz & feldspar
Feldspar, biotite,
hornblende, augite
Feldspar (pl),
augite, (olivine)
Feldspar (pl),
Augite, (olivine)
Feldspar (pl),
augite, (olivine)
xenoliths
Minerals difficult to
recognise, except
in phenocrysts
Spherulites.
Flowbanding
May be spotted.
Conchoidal
fracture.
Minerals difficult to
recognise, except
in phenocrysts
layering
Vesicles,
amygdales,
columnar jointing
Olivine plus augite
+/or hornblende
SEDIMENTARY ROCKS
MUDSTONES
AND SILTSTONES
Fossils. Mudstone
massive; shale
splits along
bedding planes
Fossils,
laminations. Cross
bedding, ripple
marks
SANDSTONES
LIMESTONE
OTHERS
Sandstone, grit and
orthoquartzite
Variable, red,
brown, green,
yellow, grey, white
Medium. Usually
well sorted. Vary
from angular (grit)
to rounded
(sandstone)
Coarse to medium,
angular
Mostly quartz. Also
feldspar, mica, rock
fragments.
Cement of calcite,
quartz, iron
minerals
Bedding, cross
bedding, ripple
marks.
Arkose sandstone
Red, pink or grey
Greywacke
Grey-black,
greenish
Sharply angular
grains (up to 2mm)
in fine matrix
Conglomerate
Variable
Breccia
Variable
Limestone
White, grey, cream,
brown
Coarse, rounded
pebbles in a finer
matrix
Angular pebbles in
a finer matrix
Variable – may be
fine-grained to
coarse
Pebbles of quartz
or flint + any rock.
matrix of sand or
silt + quartz or
calcite cement.
Calcite and some
quartz, silt or mud
Chalk
White-yellow
Calcite
Oolitic limestone
Yellow to brown
Calcite ooliths and
calcite cement
Fossils. Cross
bedding
Peat/ lignite
Brown to brown black
Carbon
Plant fragments in
peat. Bedding in
lignite
Bituminous coal
and anthracite
Black/black and
shiny
Fine, compact and
porous. Friable
Medium. Ooliths
are rounded and
well sorted
Variable. Peat
loose and friable,
lignite more
compact
Fine
Bedding. Fossils
(nb bioclastic
limestone has
abundant fossils)
Fossils and flint.
Carbon
Ironstone
Brown-yellow
Fine, medium
coarse.
Sometimes oolitic
Iron minerals, e.g.
limonite. Quartz
and calcite may be
present
Cuboid fragments
and bedding in
bituminous coal.
Conchoidal fracture
in anthracite
Beding. Cross
bedding. Fossils
Weathered granite
– often in
unconformities
above granite
Massive. Poorly
sorted graded
bedding. Slump
structures common
Bedding and fossils
rare
METAMORPHIC ROCKS
REGIONAL
CONTACT AND
REGIONAL
CONTACT
Slate
Grey, red, purple,
green
Fine, dense, may
be porphyroblastic
Diificult to identify.
Quartz, mica,
feldspar. Pyrite
porphyroblasts
Phyllite
Green-grey.
Silvery sheen
Fine to medium
shistosity
Schist
Silvery, brownish or
grey
Gneiss
Grey or pink with
darker layers
Usually medium,
but may be fine or
coarse.
Schistocity. May
be porphyroblastic
Medium to coarse.
Granular. May be
porphyroblastic
Chlorite and mica
give green/grey
colour
Mica, quartz,
feldspar. Garnet in
porphyroblasts
Metaquartzite
White to grey
Marble
White/ grey with
coloured streaks
Black, greenish or
grey with darker
spots
Black, grey , green
Spotted shale
HORNFELS
Medium to coarse.
Granular
Medium to coarse.
granular
Fine with random
spots up to 4mm
across
Fine to medium.
Porphyroblasts.
Hornfelsic
Splits into thin
sheets along
cleavage. Bedding
or fossils may be
preserved
Rocks split into
slabs
Splits into sheets.
Folds
Quartz, mica,
feldspar,
hornblende. Garnet
in porphyroblasts
Quartz (some mica
and feldspar)
Calcite (may have
dolomite)
Too fine to
recognise
Banding
(gneissose
banding). Folding
Too fine to
recognise, except
for rectangular
chiasolite
porphyroblasts or
rounded cordieraite
grains
Tough and
splintery because
of equigranular
texture
Traces of bedding
may be preserved
Traces of bedding
Traces of
bedding/cleavage