Earth and Space > Rock Identification (table)
The tables below may help the quick identification of an unknown rock type. These tables can be used alongside our Rock Identification Key to identify rarer rocks that aren’t included amongst the photographs on that page.
The tables are split into Sedimentary, Metamorphic and Igneous types.
Sedimentary
Hardness |
Grain Size |
Composition |
Other |
Rock Type |
hard |
coarse |
clean quartz |
white to brown |
Sandstone |
hard |
coarse |
quartz and feldspar |
usually very coarse |
Arkose |
hard or soft |
mixed |
mixed sediment with rock grains and clay |
grey or dark and "dirty" |
Wacke/
Graywacke |
hard or soft |
mixed |
mixed rocks and sediment |
round rocks in finer sediment matrix |
Conglomerate |
hard or
soft |
mixed |
mixed rocks and sediment |
sharp pieces in finer sediment matrix |
Breccia |
hard |
fine |
very fine sand; no clay |
feels gritty on teeth |
Siltstone |
hard |
fine |
chalcedony |
no fizzing with acid |
Chert |
soft |
fine |
clay minerals |
foliated |
Shale |
soft |
fine |
carbon |
black; burns with tarry smoke |
Coal |
soft |
fine |
calcite |
fizzes with acid |
Limestone |
soft |
coarse or fine |
dolomite |
no fizzing with acid unless powdered |
Dolomite rock |
soft |
coarse |
fossil shells |
mostly pieces |
Coquina |
very soft |
coarse |
halite |
salt taste |
Rock Salt |
very soft |
coarse |
gypsum |
white, tan or pink |
Rock Gypsum |
Metamorphic
Foliation |
Grain Size |
Hardness |
Usual Colour |
Other |
Rock Type |
foliated |
fine |
soft |
dark |
"tink" when struck |
Slate |
foliated |
fine |
soft |
dark |
shiny; crinkly foliation |
Phyllite |
foliated |
coarse |
hard |
mixed dark and light |
wrinkled foliation; often has large crystals |
Schist |
foliated |
coarse |
hard |
mixed |
banded |
Gneiss |
foliated |
coarse |
hard |
mixed |
distorted "melted" layers |
Migmatite |
foliated |
coarse |
hard |
dark |
mostly hornblende |
Amphibolite |
nonfoliated |
fine |
soft |
greenish |
shiny, mottled surface |
Serpentinite |
nonfoliated |
fine or coarse |
hard |
dark |
dull and opaque colours, found near intrusions |
Hornfels |
nonfoliated |
coarse |
hard |
red and green |
dense; garnet and pyroxene |
Eclogite |
nonfoliated |
coarse |
soft |
light |
calcite or dolomite by the acid test |
Marble |
nonfoliated |
coarse |
hard |
light |
quartz (no fizzing with acid) |
Quartzite |
Igneous
Grain Size |
Usual Colour |
Other |
Composition |
Rock Type |
fine |
dark |
glassy appearance |
lava glass |
Obsidian |
fine |
light |
many small bubbles |
lava froth from sticky lava |
Pumice |
fine |
dark |
many large bubbles |
lava froth from fluid lava |
Scoria |
fine or mixed |
light |
contains quartz |
high-silica lava |
Felsite |
fine or mixed |
medium |
between felsite and basalt |
medium-silica lava |
Andesite |
fine or mixed |
dark |
has no quartz |
low-silica lava |
Basalt |
mixed |
any colour |
large grains in fine-grained matrix |
large grains of feldspar, quartz, pyroxene or olivine |
Porphyry |
coarse |
light |
wide range of colour and grain size |
feldspar and quartz with minor mica, amphibole or pyroxene |
Granite |
coarse |
light |
like granite but without quartz |
feldspar with minor mica, amphibole or pyroxene |
Syenite |
coarse |
medium to dark |
little or no quartz |
low-calcium plagioclase and dark minerals |
Diorite |
coarse |
medium to dark |
no quartz; may have olivine |
high-calcium plagioclase and dark minerals |
Gabbro |
coarse |
dark |
dense; always has olivine |
olivine with amphibole and/or pyroxene |
Peridotite |
coarse |
dark |
dense |
mostly pyroxene with olivine and amphibole |
Pyroxenite |
coarse |
green |
dense |
at least 90% olivine |
Dunite |
very coarse |
any colour |
usually in small intrusive bodies |
typically granitic |
Pegmatite |
Related Links
Rock Identification Key
Rock Types