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) |
||
|
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
|
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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