1.
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Rocks & Minerals
ISSN: 0035-7529 (Print) 1940-1191 (Online) Journal homepage: http://www.tandfonline.com/loi/vram20
A Practical Guide to Rock Identification
Albert J. Copley
To cite this article: Albert J. Copley (1973) A Practical Guide to Rock Identification, Rocks &
Minerals, 48:12, 728-731, DOI: 10.1080/00357529.1973.11763375
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2.
A PRACTICAL GUIDE TO
By ALBERT
Northeast Mo. State Univ,
When the student embarks on this
study it is presumed he is familiar with
cleavage, fracture and hardness determina­
tions of minerals. These properties as
used in mineral identification are not
strictly valid in rock study. However,
these properties can be used to advantage
if it is realized that a rock is an aggre­
gate of separate mineral grains. If most
of the grains in a rock have a hardness
in the same range, then a hardness de­
termination will be of value in identify­
ing the rock. With this in mind the fol­
lowing set of descriptive sheets can be
utilized for rock identification. It should
be noted that percentages of minerals
within a rock may vary within certain
limits depending upon the rock in ques­
tion. Therefore certain rocks may vary in
hardness, and are in some instances listed
in two different places in the study guide.
Please keep in mind also, you may en-
counter a rock which is completely atypi­
cal and fails to fall within the limits set
up within this study guide. Please rea­
lize also, that hardness and friability are
distinct properties. Be sure you are de­
termining hardness of the constituent
minerals and not crumbling properties of
the rock.
HARDNESS
A. W I L L NOT SCRATCH GLASS
Gypsum:
A chemical precipitate consisting large­
ly of the mineral gypsum. Colors are
commonly whitish, pink, grey, buff.
Shale:
This is essentially compacted mud. It
commonly shows layering, but may also
be blocky or chunky. It may be calcar­
eous if it reacts to hydrochloric acid; its
softness is the diagnostic feature to sep­
arate it from limestone. It may also con­
tain amounts of sand or organic material,
fossils may be present. Chew on it light­
ly and it will mud up. Fastidious people
may add a drop or two of water. Mud-
ding and hardness will separate it from
slate. Colors are generally grey, red,
black. Light colors occur but are not so
common (green, blue grey, buff, white).
728
ROCK IDENTIFICATION
J . COPLEY
., Kirksville, Mo. 63501
Coal:
This is carbonaceous material derived
from plant material. Its light weight
and softness will serve to distinguish it
from obsidian which it can resemble. It
is regarded as a sedimentary rock by many
people. Consider the series: woody tis­
sue, peat, lignite, bituminous coal, anthra­
cite. Anthracite is essentially pure car­
bon and is found only where considerable
folding has occurred. Therefore some
purists consider anthracite coal to be a
metamorphic.
Schist:
A metamorphic rock which is finely
foliated. Schists differ from gneiss in
having less feldspar. Quartz and mica
are usually present. If enough chlorite
or talc is present you may overlook the
presence of quartz (if present) and there­
fore be looking in this soft rock category.
B. HARDER T H A N FINGERNAIL
Limestone:
Most limestone is a biochemical pre­
cipitate. It is essentially pure Calcite and
as such will have most of the properties
of Calcite. Usually some shale or sand­
stone will be admixed with resulting
changes in colors and other properties.
Common colors are grey, black, pink,
buff. A vigorous reaction to dilute Hy­
drochloric acid is diagnostic. Fossils or
fossil fragments are common.
Travertine:
A fresh water deposit of calcite, usual­
ly found in caves and springs. Common­
ly it shows a layering. It is difficult to
separate from limestone. It actually is
a type of limestone.
Dolomite:
This is a limestone which is rich in
magnesium carbonate. It has the same
properties as limestone except that fos­
sils are not so common and its reaction
to dilute hydrochloric acid is so subdued
as to be almost unrecognizable. Powder
a bit of the rock and put a drop of acid
on the powder; close examination is re­
quired to really be sure if it is reacting
or merely absorbing the liquid as a dry
powder will do.
ROCKS A N D M I N E R A L S

3.
Marble:
This rock is metamorphosed limestone
or dolomite. As such it has the properties
of the original rock. Close examination
with acid may be necessary to distinguish
a dolomitic marble. In the metamorphos­
ing process fossils present will be de­
stroyed. Also, a crystalline texture will
be developed. Colors can be varied de­
pending upon the original rock and what­
ever impurities are present. Some marble
may contain small amounts of minerals
hard enough to scratch glass. Use care in
determining hardness of a rock suspected
of being marble. A careless visual ex­
amination will sometimes result in con­
fusing marble with quartzite. Hardness
testing will prove conclusively which it
is.
Coal:
This rock was discussed in the group
which is softer than fingernail. A high
grade anthracite coal may be hard enough
to put it in this group. A match or lighter
applied to a small crumb will identify
coal. In any case it will not scratch glass
and there should be no confusion with
obsidian, a volcanic glass.
W I L L SCRATCH GLASS
A. OBVIOUS FOLIATION OR
LAYERING PRESENT
Gneiss:
This is a metamorphic rock which is
typically banded with alternating layers
of rhe light-colored minerals, quartz and
feldspar and dark colored mica and horn­
blende. Other minerals may or may not
be present.
Schist:
These metamorphic rocks differ from
gneisses in being more finely foliated and
in lacking large auantities of feldspars.
The presence of a predominant mineral
is usually noted by using its name as a
qualifying adjective. Examples would be
hornblende Schist, Quartz Schist, etc. Col­
ors will depend upon the minerals pres­
ent. Common minerals are chlorite, talc,
graphite, and pyroxene.
Slate:
This is metamorphic rock formed from
shale. Visually its properties will be much
like shale. Its hardness will be much
greater, however. Water added to it will
not cause it to mud up. Slates have a
cleavage which allows them to split into
broad thin sheets.
B. NO OBVIOUS FOLIATIONS PRESENT
1. EXTREMELY POROUS OR VESICULAR
Scoria congealed volcanic lava, colors
are usually grey, black, red.
Pumice:
Pumice is a lightweight volcanic glass
which has been frothed Hghtly as whipped
cream and then frozen. Bits of Scoria
will be opaque on close examination.
Pumice, however, will reveal a glassy na­
ture. Pumice may be lightweight enough
to float on water.
2. GLASSY T E X T U R E
Obsidian:
Volcanic glass which may be of various
colors: black, grey, red, golden. A large
piece may show flow banding. If it has
devitrified enough it may look like a lump
of anthracite coal or pitch. One variety
is termed pitchstone. Hardness and spe­
cific gravity will immediately serve to dis­
tinguish obsidian.
3. FINE GRAINED:
This term is used to indicate a grain
size fine enough that only a microscope
will distinguish individual grains,
a. LIGHTCOLORED-FINE GRAINED
Quartzite:
Metamorphic equivalent of sandstone
and as such essentially pure quartz, felds­
par, mica and chlorite may be present.
The hardness and vitreous (glassy) lus­
ter of quartz are the key to separating
it from felsite.
Felsite:
This is a "waste-basket" term which
may be applied to igneous rocks which
are light-colored, and fine grained. Rhyo-
lite and trachyte would be included in
this grouping. Large pieces of these rocks
may show a flow banding. They may
be porphyritic. There will be enough
feldspar nresent that the luster will not
be vitreous.
Tuff:
This is a cemented volcanic ash. It is
somewhat granular and can resemble a
sandstone, which it is, in one sense of the
word. That is, if one is using "sand­
stone" to mean grain size and not com­
position. Tuff has also been transported
by a fluid medium, air and consequently
may be layered in large pieces.
ROCKS A N D M I N E R A L S 729

4.
Chert:
This is a sedimentary rock commonly
known as flint. Usually it contains bits
of fossil fragments which coupled with its
fine grain size will serve to identify it.
It might be oolitic or banded. It is sili­
ceous material and will have conchoidal
fracture as quartz. Fracture and dull vit­
reous luster should serve to distinguish
it from some varieties of felsite.
b. DARK-COLORED. FINE-GRAINED
Basalt:
This is an igneous rock term which
includes both basalt and andesite which
are fine-grained equivalents of gabbro and
andesite respectively. These rocks are
dark grey to black and are composed
mostly of pyroxene, olivine, plagioclase
and hornblende. Visually some varieties
of black limestone resemble basalt. The
softness, reaction to acid, and presence
of fossils in limestone will immediately
distinguish it.
4. COARSE-GRAINED
A. COARSE-GRAINED LICHT COLORED
Conglomerate:
A sedimentary rock composed of
rounded gravels held together by some
cementing agent such as silica, calcium
carbonate, or iron oxide. Usually there
is a considerable size difference from the
largest to the smallest size of particles.
Breccia:
These rocks are much like conglom­
erates except that fragments are angular
instead of being rounded. The signifi­
cance is that the particles have not been
transported any great distance and hence
are not rounded.
Sandstone:
This sedimentary rock was entered in
the fine-grained section. Sandstones may
range in size from rather fine grains to
coarse. Keep in mind that composition
of grains of sand is the diagnostic fea­
ture. Do not look for any particular color
or cementing agent. Sandstones are com­
monly cemented together with silica, iron
oxide or calcium carbonate.
Arkose:
This is actually a special type of sand­
stone. It will have unaltered or only
slightly altered particles of feldspar, and
be different in that respect.
Porphyry:
An igneous rock which contains rela­
tively large crystals embedded in a fine-
grain ground mass. The significance is
that two distinct cooling rates were im­
portant in the formation of the rock. A l ­
most any type of igneous rock may be
porphyritic. Keep looking to try to de­
termine what kind of porphyry is under
study, for example, felsite porphyry, ba­
salt porphyry, etc.
Chert:
This sedimentary rock is listed also un­
der fine-grained rocks. It may vary ex­
tremely in appearance. Some chert is
easily confused with a conglomerate or
breccia. Study the particles carefully.
Also, consider the enclosing material to
determine if it is chert or flint or merely
small quantities of silica cement.
Tuff:
Some tuffs are composed of rather large
particles and can be confused with sand­
stones. The particles composing a tuff,
however, should show either crystal boun­
daries or show a glassy texture. This is
of volcanic pyroclastic origin.
Pegmatite:
This igneous rock is actually a special
type of granite. The point at which gran­
ite begins to be called pegmatitic is not
definite. A pegmatite, however, is com­
posed of such large crystals of quartz
and feldspar that a hand sample must be
carefully selected to show several minerals.
Granite:
An igneous rock composed essentially
of quartz and feldspar. Color will depend
upon which feldspar is present. Mica and
hornblende may be present in small
amounts.
Syenite:
This igneous rock is much like granite,
however, is different in that syenite con­
tains little or no quartz.
B. COARSE-GRAINED, DARK-COLORED
Diorite:
This is actually an intermediate igneous
rock. Color tones may vary somewhat
like granite to darker as gabbro. Diorite
is composed of plagioclase feldspar with
hornblende and biotite. Some orthoclase
and quartz will be present.
730 ROCKS A N D M I N E R A L S

5.
Gabbro:
This rode may be extremely dark gray
or dark green or black. It is composed
chiefly of pyroxene and plagioclase felds­
par. Olivine may be present. If in large
amounts it may impart a greenish color
to the rock.
Peridotire:
This igneous rock consists mostly of
ferromagnesian minerals. Minor amounts
of feldspar may be present. Olivine may
be present. Magnetite is frequently pres­
ent in quantities large enough to be
picked up by a magnet.
WORKSHOP 74
Want to learn more about carving?
Micromounts? Tumbling? Cleaning min­
erals? Silversmithing? These, and most
all other phases of mineralogy, lapidary,
paleontology will be demonstrated at
Workshop '74, sponsored by the Gem
and Mineral Society of Syracuse, Inc.
The two-day affair will be held Feb­
ruary 16-17, 1974, in the Syracuse Auto
Auction Building, located on Route 11
just south of Syracuse (between Nedrow
and Lafayette). This is the site of Gem-
World, the club's annual show held each
July. Hours of the Workshop will be
Saturday, 10-7, and on Sunday, 10-5. It
is open to anyone interested in any phase
of the hobby.
Most who participated in Workshop
'73 are expected to again display their
talents. In addition to the topics men­
tioned above, there will be demonstra­
tions of faceting, cabochon-cutting (in­
cluding opal), inlay, lost wax casting, and
using baroques in various ways. It is
expected that there will again be exhibits
of minerals from micromounts to cabinet
size, fossils, and lapidary of various types.
Anyone who did not take part in past
workshops would be most welcome to
either display or demonstrate at Work­
shop '74, whether it be for the full two
days, for one day, or even part of a day.
The object is to teach each other—and
the general public—all we can about the
hobby. Workshop '74 is strictly non­
commercial and no selling will be per­
mitted on the site.
Another feature will be a swap session
to be held during the Workshop. A
large section of the building will be set
iside for this purpose. It is hoped that
many rockhounds will come to swap. Es-
ROCKS A N D M I N E R A L S
cape from winter's "cabin fever" for at
least a couple days! Bring your swapping
materials and your own table to display
your "goodies" on.
No admission will be charged. How­
ever, each swapper and exhibitor will be
asked to donate a specimen or other item
related to the hobby for the auction.
There will be one auction each afternoon
to help defray expenses of the Workshop.
Refreshments will be available through­
out both days.
Chairman of Workshop '74 is Bill
Bethard with Gene Ridall co-chairman.
Any inquiries about Workshop '74 or
the swap should be addressed to Mr.
Bethard, care of Gem & Mineral Society
of Syracuse, Inc., P. O. Box 801, Syra­
cuse, New York 13201.
THE "NO W A S H " METHOD
OF TUMBLE POLISHING
Start with No. 80 grit, tumble a week.
Don't empty tumbler; but add five tea­
spoons full of new grit to the mixture
in a three-pound tumbler. Follow this
procedure through with No. 190, 320 and
600 grits. Thoroughly wash the stones
and tumbler before polish and final
powders.
For a glossy finish, after washing out
add three spoons of sugar, one level spoon
of Cascade or All or any non-sudsing de­
tergent, and about ten drops of muriatic
acid if you have some. Let stand open
one minute, close tub and tumble for a
week.
Rocky Mt. Fed. Newsletter
via the AFMS Newsletter
731