How to Identify Different Types of Rocks?

Rocks Identification

These tables will
help you identify almost any rock you’re likely to find. First determine
whether your rock is igneous, sedimentary or metamorphic. The quick way to tell
is:

  • Igneous: little texture or
    layering; mostly black, white and/or gray minerals; may look like lava (About
    Igneous Rocks
    )
  • Sedimentary: layers of sandy or
    clayey rock (strata); tends to split along layers; mostly brown or gray;
    may have fossils and water or wind marks (About
    Sedimentary Rocks
    )
  • Metamorphic: layers of light and
    dark minerals (foliation), often wavy; various colors; glittery from mica
    (About
    Metamorphic Rocks
    )

Then start in the
left column of the appropriate table and work your way across. Follow the links
to pictures and more information. If you don’t find a match, try another of the
three big types.
Grain Size has two values:
“coarse” grains are visible, and the minerals can usually be
identified using a magnifier; “fine” grains are invisible to the
naked eye and cannot be identified with a magnifier. The cutoff is about 0.1
millimeter, between sand size and silt size. (using a
magnifier
)
Hardness has three values:
“hard” rock scratches glass, which usually signifies the minerals
quartz or feldspar (Mohs hardness
6-7 and up); “soft” rock does not scratch glass but scratches
fingernails (Mohs 3-5); “very soft” rock can be scratched with a
fingernail (Mohs 1-2). All igneous rocks are hard.


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Igneous
Rocks

Grain
Size
Color
Other
Composition
Rock
Type
fine
dark
glassy
texture
lava
glass, usually felsic
fine
light
frothy
texture
lava
froth, felsic
fine
dark
frothy
texture
lava
froth, basaltic
fine
light
contains
quartz
high-silica
(felsic) lava
fine
medium
between
felsite and basalt
medium-silica
lava
Andesite
fine
dark
has
no quartz
low-silica
(mafic) lava
mixed
any
color
fine
groundmass with larger grains (phenocrysts)
feldspar,
quartz,
pyroxene or olivine
phenocrysts
coarse
light
wide
range of color and grain size
feldspar
and quartz with minor mica, amphibole or pyroxene; felsic
coarse
light
like
granite but without quartz
feldspar
with minor mica, amphibole or pyroxene; felsic
Syenite
coarse
medium
to dark
little
or no quartz
feldspar
and dark minerals; mafic
coarse
medium
to dark
no
quartz; may have olivine
feldspar
and dark minerals; mafic
coarse
dark
dense;
always has olivine
dark
minerals, pyroxene and olivine; ultramafic
Peridotite
coarse
dark
dense
pyroxene;
ultramafic
Pyroxenite
coarse
green
dense
olivine;
ultramafic
Dunite

Sedimentary
Rocks

SEE ALSO:  Erosion - Types, Phases, Agents, and Effects
Hardness
Grain
Size
Composition
Other
Rock
Type
hard
mixed
mixed
rocks and sediment
round
rocks in sediment matrix
hard
or
soft
mixed
mixed
rocks and sediment
sharp
pieces of same material as groundmass
hard
coarse
clean
quartz
white
to brown
hard
coarse
clean
quartz and feldspar
usually
very coarse
Arkose
hard
mixed
mixed
sediment with rock grains and clay
gray
or dark and “dirty”
Graywacke
hard
fine
very
fine sand; no clay
feels
gritty on teeth
hard
fine
no
fizzing with acid
soft
fine
clay
minerals
foliated
soft
fine
carbon
black;
burns with tarry smoke
soft
fine
fizzes
with acid
soft
coarse
or fine
dolomite
(magnesian calcite)
no
fizzing with acid unless powdered
Dolomite
or Dolostone
soft
coarse
fossil
shells
mostly
pieces
Coquina
very
soft
coarse
salt
taste
very
soft
coarse
white
or pink
Gypsum

Metamorphic
Rocks

Foliation
Grain
Size
Hardness
Color
Other
Rock
Type
foliated
fine
soft
dark
“tink”
when struck
foliated
fine
soft
dark
shiny;
crinkly foliation
foliated
coarse
hard
mixed
“salt & pepper”
often
has large crystals
foliated
coarse
hard
mixed
banded
foliated
coarse
hard
mixed
distorted
“melted” layers
foliated
coarse
hard
dark
mostly
hornblende
Amphibolite
nonfoliated
coarse
hard
red
and green
dense;
garnet and pyroxene
nonfoliated
coarse
soft
light
fizzes
with acid (calcite)
nonfoliated
coarse
hard
light
no
fizzing with acid (quartz)
nonfoliated
coarse
or fine
hard
dark
dull
and opaque colors
Hornfels

———-
For correction and improvements please use the comments section below.


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