It is not easy to tell the difference
between rocks & minerals because there are so many kinds of them. It takes
years of study to be able to accurately identify a mystery rock and even
then rockhounds want to know where the specimen came from. For more
information see How Rocks & Minerals are
All rocks are made of 2 or more minerals,
but minerals are not made of rocks.
There are many common names for rocks and the usually give you an idea of how
big the rock is. Here are a few:
- mountain - huge, giant hunk of rock that is still attached to the
earth's crust, doesn't move, tall
- boulder - large, taller than a person
- rock - large, you could get your arms around it or a bit smaller
but it is usually jagged,
broken off a bigger piece of rock
- river rock - round rocks that are along the edge & at the
bottom of fast-flowing rivers
- stone - medium, you could hold it in two hands
- pebble - small, you can hold it with two fingers, could get stuck
in your shoe, usually rounded
- sand - made up of tiny pieces of rock, grains of sand
- grain - tiny, like a grain of rice or smaller, often found on a
- dust - really fine powder that is mixed in with sand or soil
- speck - as in a speck of dirt
- A mineral is the same all the way through.
That is one reason we speak of
a sample or a specimen rather than a
- There are about 3000 known minerals on earth.
- All rocks are made up of 2 or more of these
For a good explanation of the difference between
rocks & minerals, check out http://www.rocks-and-minerals.com/
Books to help you identify rocks & minerals
Most rockhounds start out by just looking
at rocks and getting to know them. But after a while, you're going to want to
Field Guides are a great source of information. There
are a number of other field guides available including specific guides to fossil
and gemstones. Some excellent field
- Simon & Schuster’s Guide to Rocks and Minerals
(ISBN 0-671-24417-5), my personal favourite.
- Eyewitness Handbooks Rocks & Minerals
(ISBN 0-7737-2575-X), a fabulous book for the
ROCKS & MINERALS
For kids, there are a variety of books on rocks &
minerals. Some really good ones are:
Books Rocks & Minerals (ISBN
Looking at pictures of rocks & minerals & getting familiar with what
they look like will also help you identify minerals. For photographs on the web
of minerals, check out the Mineral Identification web sites listed below.
Characteristics used in the
identification & study of minerals. These are the most common
characteristics used when describing minerals.
- Colour – this varies depending
on the chemicals present and is the least informative in identifying a
- Luster – what the surface
looks like in the light
- Specific Gravity – how heavy
it feels, heft
- Crystal Form –
shape of crystal, shape the mineral would take if it had room to grow in a cavity,
massive – some minerals have a number of different crystal shapes
- Cleavage – pattern when
mineral is broken – in planes or conchoidal
- toughness, how cohesive the
mineral is, if it falls apart
- Hardness – what it can scratch
& what scratches it
- Transparency - The ability to transmit light. Depending on a number
rocks & minerals can also transmit light.
Many rocks that are
opaque when in a chunk, are translucent when cut into very thin slices.
stones are often valued on how clear, or transparent they are.
- Special Properties– magnetism, chatoyancy, fluorescence, odor, streak, burn test,
Words used to describe the way light
reflects off of the surface of a mineral:
|dull / earthy
||very dull, mainly in minerals that
||like the surface of a candle
|greasy / oily
||like a pearl, play of colors,
||talc, muscovite mica
||has a shiny surface like a piece
of silk cloth
||some varieties of gypsum, kernite,
ulexite & in fibrous minerals
|glassy / vitreous
||looks like glass
||quartz, many rock-forming
minerals, obsidian – "nature’s glass"
||looks like freshly-broken shellac,
||high luster, almost brilliant,
||silvery or metallic luster but
mineral is transparent or translucent when in small slivers
||very shiny, like processed metals,
highly reflective, opaque minerals
||pyrite, gold, silver
Other words that
- shiny, sparkly
- shimmering, opalescent
- frosted, milky
|Specific Gravity (SG) indicates how many
times more the mineral weighs
compared to an equal amount of water (SG 1).
- So if you have a bucket of silver, it would weigh 10 times as much
as a bucket of water.
If you have a bucket of calcite, it would only
weigh about 2 1/2 times as much as a bucket of water.
That is why we
think of metals as being "heavy".
They are heavy compared to other
things that we are used to picking up.
This is also known as the "heft"
of an object.
- The average rock you would pick up has an SG of about 2.75
most of the earth’s crust is made up of quartz, calcite & feldspar.
- When something feels heavy, it feels heavier than expected for
something of that size.
How hefty a specimen feels has to do with how
dense it is, its mass compared to its volume.
||2 - 2.5
||gypsum, halite, selenite, ulexite
||2 – 3
||calcite, dolomite, feldspar,
muscovite mica, quartz, talc, turquoise,
|above average / slightly heavy
||3 - 4
||4 – 5
||almandine garnet, apatite, barite,
celestite, chalcopyrite, fluorite
||5 – 10
|| galena, hematite, magnetite,
|extremely heavy even for a
||must be platinum!
|Gemstones are sold by weight. A chart that
relates to the specific gravity of gemstones can be found at:
Because of specific gravity, that means that 2 different gems that are
the same size weigh different amounts.
When minerals have the time & space to grow into their crystal forms,
they grow to beautiful regular shapes that are easy to recognize once you
have seen a few examples.
Some words used to describe crystal forms or
- acicular / radiating needles ~
crystals that grow in fine needles
~ rounded blobs
- botryoidal ~ looks like top of
bunch of grapes
~ spherical, round shape that is solid, the same all the way through or
filled with layers or agate
- cubic ~ 6 equal, square faces
- dendritic ~ branching,
tree-like, looks like the veins in a
leaf or like a painted “tree shape”
dodecahedron ~ 12 sided, like a 12 sided die
- dog-tooth ~ shaped like the
canine tooth, like a dog's tooth
~ looks like fibers, threads, parallel lines
~ spherical, round shape that is hollow inside, often lined with
- hexagonal prism with pyramid termination
~ hexagonal cross-section, with pointy ends (terminations)
prism with rounded ends ~ 6 sided cross-section, with rounded
pyramid ~ sharp 6 sided pyramid, often seen in clusters
~ rounded like botryoidal but a bit bigger than a bunch of grapes
~ a chunk of mineral with no crystal shape evident
- octahedral ~ 8 sided
- prismatic ~ like a prism
with flat ends, longer than it is wide
pyritohedral ~ 12 sided with 5 sided pentagon faces
- rose shaped
~ looks like a flattened flower or rose with petals
- tabular ~ divide easily into
thin plates or sheets, a stack is know as a “book”
~ the end of a complete crystal
Cleavage is when a mineral breaks with smooth flat surfaces.
can be described as perfect, good, imperfect,
It can also be described as:
- Perfect 1 way ~ breaks on one
perfect cleavage plane, crystals break into slices, sheets peel off
- Perfect 2 ways ~ breaks into
elongated boxy shapes, 90 degree angles
- Perfect 3 ways ~ breaks into
perfect rhombs, pieces look like squished boxes
- No cleavage ~ does not break
Fracture is when a mineral breaks, but the surface is
not regular, does not show cleavage.
Words that describe what a break in a rock or mineral looks like:
- conchoidal ~ curved break like
what happens with thick glass or bottle bottom, shell shaped,
can be rough or smooth
- jagged ~ metals, sharp point
that scratches or snags fingertips, hackly
- splintery ~ fibrous
- uneven ~ rough surface, not
Tenacity is how tough a
mineral is, how easily a mineral will break,
split, crumble or change shape.
Terms used to describe this trait are:
elastic ~ can
be bent & when let go they resume their previous shape ~ mica
ductile ~ can
be pulled to make very thin threads ~ gold
fragile ~ break
into pieces easily
flatten out into thin sheets without breaking ~ gold
sectile ~ can
be cut with a blade to make shavings ~ gypsum
MOHS SCALE OF HARDNESS - a scale devised by
- fingernail (2.2)
- copper penny (3.5)
- pocket knife or common nail (5.2)
- piece of glass (5.5)
- steel file or concrete nail (7.5)
- piece of corundum (9)
Notes for testing:
- Each mineral can scratch the minerals with
lower hardness ratings.
- Each mineral can scratch itself.
- Don’t press hard, normal scratching should do.
- Weathered surfaces are softer.
- Corners or edges of crystals are softer.
- Small pieces seem softer than large pieces.
- When you scratch, take a close look at the
scratch line -
which often looks white.
Is it really a scratch or is it
a powder line made from the tool you used
because it was softer than the
item you were trying to scratch?
||Easily crumbles. Can be scratched with a fingernail (2.2)
||Can be scratched with a fingernail (2.2)
||Can be scratched with a copper penny (3.5)
||Can be scratched with a common nail (5.2)
||Can be scratched with a common nail (5.2).
B. Mineral of hardness 6 or more will scratch glass.
||Can be scratched with a concrete nail (7.5).
||Used in industrial tools
for cutting, grinding & sanding.
||Diamond is used to cut all minerals including diamonds.
For a more detailed article about HARDNESS,
site that discusses hardness in gems and minerals can be found at
|A mineral can be:
- clear, see right through it when
it is sliced thin
- called "gemmy",
desirable for gemstones
quartz (rock crystal),
- see shapes & shadows through
it when it is sliced thin
- chemical impurities can cause
the mineral to be cloudy
|calcite, quartz, sphalerite
- can’t see light through it at
all when it is sliced thin
- rarely used for gemstones
of Words & Phrases
acicular / radiating
needles ~ crystals that grow in fine needles
very shiny like a gemmy crystal, almost brilliant
looks like top of bunch of grapes
shines like a cat’s eye because of fibers
~ everything on earth is made up of the 103 known chemical elements,
including rocks, mineral,
air, water, plants & animals
cleavage ~ the property to
break along smooth lines or planes, the mineral has a shape it wants to be &
breaks along those lines to keep that shape
conchoidal ~ curved break
like what happens with thick glass or bottle bottom, shell shaped, can be
rough or smooth
concretion ~ spherical
mass that is separate from the rock around it, usually weathers out of host
grows from the inside out
crystal shape ~ the form
or habit of a mineral, the shape that the mineral takes if it has the time &
to grow properly
~ minerals that form slowly have a distinctive crystal shape
cubic ~ 6 equal, square
dendritic ~ branching,
dog-tooth ~ shaped like
the canine tooth, like a dog's tooth
dull / earthy ~ very dull,
mainly in minerals that are porous
~ the earth's crust is made of solid, hardened rocks & minerals
erosion ~ the process
through which mountains are broken down into boulders & sand
fossil ~ the remains of
plants & animals that have been replaced by minerals
fracture ~ is the way a
mineral breaks when it won’t break on a cleavage plane
~ rocks & minerals that have been cut & polished, used for decoration
and are usually rare and
geode ~ a sphere with a
hollow inside, often lined with crystals, grows from the outside in
geologist ~ a scientist
that studies rocks & minerals and the earth sciences
glassy ~ shiny like glass,
found in 70% of minerals, vitreous
hardness ~ how easy it is
to scratch a mineral
hexagonal prism with pyramid
termination ~ hexagonal cross-section, with pointy ends
igneous ~ rocks made from
fire & heat, liquid magma that has cooled to form rocks
luster ~ how shiny
something is; words used to describe the way light reflects off of the surface
of a specimen
massive ~ a mineral with
no distinct crystal shape, large chunk of inter-grown minerals
matrix ~ the host rock
that a mineral specimen or crystal is found in or on, bedrock
metallic ~ shiny like
polished metal, highly reflective, usually opaque
metamorphic ~ igneous or
sedimentary rocks that have been changed through extreme heat &
to movement of the earth’s crust
mineral ~ non-living
matter, chemically the same all the way through
minerals ~ all rocks are made of one or more of the 3000 known minerals
no cleavage ~ does not
opaque ~ cannot see
through it at all, blocks all light, casts a solid shadow, acts like a wall
paleontologist ~ a
scientist who studies paleontology, learning about the forms of life that
existed in former
geologic periods, chiefly by studying fossils
pearly ~ like a pearl,
play of colors on surface
physical properties ~
the common visible and tangible characteristics used in the identification &
rockhound ~ a lover of
rocks, minerals & fossils who collects specimens in the field
rocks ~ non-living matter,
made of 2 or more minerals
~ rocks are constantly forming, wearing down and forming again, very slowly
schiller ~ colors shimmer
or flash when the light hits the surface in a certain way
sedimentary ~ layers of
sand, clay & bits of rock laid down by water & turned to rock, often contains
specific gravity ~ how
heavy something feels when compared to what you would expect,
sub-metallic ~ soft shine
like dull metal
tabular ~ divide easily
into thin sheets, a stack is know as a “book”
termination ~ the point at
the end of a crystal
translucent ~ see shadows
and shapes through it when held up to the light, details not clear, is frosted
cloudy, like looking through wax paper or light
transparency ~ describes
if you can see through something or not
transparent ~ clear, see
through clearly all the way, like a plain window glass or clear plastic wrap,
“gemmy” like a gemstone
uneven fracture ~ rough
surface, not smooth
vitreous ~ shiny like
glass, found in 70% of minerals, glassy
waxy ~ looks softly shiny
like wax, like the surface of a wax candle
For a Geological Dictionary go to http://www.geologyshop.co.uk/dictio~1.htm
Mineral Field Tests -
Test to perform while collecting
Mineral Field Tests – or Tests on the Go
© Bert Ellison 1999
Trouble identifying that precious piece that you tapped out of the quarry or
dump? Don’t despair, if you’ll settle for a rough field estimate, but you’ll
have to resort to more sophisticated tests if you want to be precise. So these
are field tests only:
– is it flat and scaly like mica or in a crystal form?
Crystals of unusual size and shape are rare – they are in museums! Since
there are some thirty variations of the crystal systems, few of us are
qualified to judge. A poor field use, though some forms are useful e.g.
Colour – useful in some cases but not reliable. Beware of
oxidation or tarnish which hides the true colour. Also many minerals come in
many hues e.g. quartz, calcite.
Lustre – the way light is reflected from a mineral. Of very little
– or transparency. Metallic minerals are opaque. Some
transparent ones may be potential gemstones. Beyond sorting out the metallics,
this property is of limited use but don’t toss away any emerald, topaz, ruby
Specific Gravity – or SG is of some use. Most metallics run about SG
3 to 4. "Stony" minerals are about 2 to 3. If it comes in over 6,
stake a claim! Most dumps don’t offer specimens large enough to
"heft" for us to judge. Use at least a good "thumbnail"
Streak – press a piece across unglazed tile & note colour of
powder. Very useful! Cuts through tarnish.
Hardness – or H. Get to know Mohs scale! This is a very useful
quality and usually the first test one makes. Keep that knife handy!
While many minerals may be similar, this test is great for sorting out the two
great stoney groups – calcite/limestone etc. and quartz. Hardness alone may
at least put you on the right track. Good for metallics too – try pyrite vs.
gold (6.5 vs. 2.5). Excellent first test but some minerals are harder in
certain directions. E.g. kyanite; 4-5 lengthwise, 6-7 across the crystal.
Cleavage – not the burlesque type but the way a piece breaks.
Shell-like (conchoidal) yields sharp shards (as flint). Some yield smooth flat
breaks (as micas) and some are partly smooth & rough in different
directions (as feldspars). The quartz group – chert, flint, amethyst etc.
– have very rough breaks. So do garnets. Of modest use but good for
feldspars, quartz, micas, calcite, galena and halite for examples.
Acid – use 10% HCl (hydrochloric or "plumbers" acid) in
squeeze bottle. Excellent to verify the carbonates from almost anything else,
especially the quartz family. Great for limestone vs. dolomite. Fizzes slowly
on cold rock. Warm it up first.
Oddballs – Taste – don’t lick everything – there is lots of
arsenic around! Great for halite and potash salts if you suspect them.
Oddballs – Magnetic – very useful for picking out magnetite,
ground-up pyrrhotite (an iron sulfide). Use a horseshoe magnet suspended on
Oddballs – Fluorescence – of some use (in the dark) for fluorite,
some calcite, scheelite and sphalerite .. Oh yes! And diamonds too!