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THE DIFFERENCE BETWEEN ROCKS & MINERALS
Identifying Minerals. 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 Formed
All rocks are made of 2 or more minerals, but minerals are not made of rocks.
Rock Words: 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 beach
- dust – really fine powder that is mixed in with sand or soil
- speck – as in a speck of dirt
For an excellent Rock Identification Test, click on this link – http://www.bwctc.northants.sch.uk/Learning/Science/Rocks/
- A mineral is the same all the way through. That is one reason we speak ofa sample or a specimen rather than a rock.
- There are about 3000 known minerals on earth.
- All rocks are made up of 2 or more of these minerals.
For a good explanation of the difference between rocks & minerals, check out other related websites.
Identifying Minerals FIELD GUIDES – 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 know more. 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 guides are:
- (ISBN 0-671-24417-5), my personal favorite.
- Eyewitness Handbooks Rocks & Minerals (ISBN 0-7737-2575-X), a fabulous book for the beginning rockhound.
BOOKS ON ROCKS & MINERALS
For kids, there are a variety of books on rocks & minerals. Some really good ones are:
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 websites listed below.
- Mineralogy Database
- Mineral Identification – Maintained in Australia (Website discontinued)
- Minerals by Class – Maintained by Amethyst Galleries Inc.
- Minerals by Name – Maintained by Amethyst Galleries Inc.
PROPERTIES OF MINERALS
Characteristics used in the identification & study of minerals. These are the most common characteristics used when describing minerals.
- Color – this varies depending on the chemicals present and is the least informative in identifying a mineral variety
- 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,
- Not massive – some minerals have a number of different crystal shapes
- Cleavage – pattern when mineral is broken – in planes or conchoidal
- Tenacity – 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 of things,rocks & minerals can also transmit light.
- Many rocks that are opaque when in a chunk, are translucent when cut into very thin slices. Gems stones are often valued on how clear, or transparent they are.
- Special Properties– magnetism, chatoyancy, fluorescence, odor, streak, burn test, conductivity
Although most people think of color as an important characteristic of a specimen, it is not very useful in identifying a mineral.
- color is one of the physical properties most commonly used to describe minerals,but it is not a very good property to use to identify minerals
- some minerals are nearly always the same color like azurite (blue) and sulfur (yellow)
- many minerals come in a variety of colors – the changes are caused byslight chemical impurities or through exposure to heat
- color can change when the surface is exposed to moisture & air – it tarnishes or oxidizes
- some minerals have common names (varietal names) that describe a specimenwith a certain color eg. Quartz – rock crystal (colorless), smoky quartz (brown), citrine (yellow), amethyst (violet), rose quartz (pink)
- color can be described as metallic or non-metallic and is often described along withluster though they are 2 different characteristics
- rocks are often distinctive or named because of a certain color which occurs because oftheir mineral content
|Color Grouping||Rock & Mineral Example|
|golden, golden yellow||gold, pyrite|
|bronze||chalcopyrite, nickel ore|
|silver, silvery-yellow, silvery-gray, bright silver, dark silver, black||antimony, galena, manganite, silver|
|peacock feathers, rainbow||bornite, peacock ore|
|colorless||barite, quartz, rock crystal, selenite|
|white, beige, creamy, dirty-white, snow white||calcite, gypsum, muscovite mica, quartz, stilbite, talc|
|yellow, orange, brown, browny-beige||barite rose, calcite, cancrinite, celestite, jasper, siderite, sphalerite, , sulfur|
|green, turquoise green, moss green, lime green||amazonite feldspar, apatite, bloodstone, emerald, epidote, fluorite, grossular garnet, jade, malachite, turquoise|
|blue, sky blue, turquoise blue, pale blue, steel blue, deep blue||azurite, celestite, kyanite, labradorite, lapis lazuli, sodalite, turquoise|
|red, purple, maroon, violet, mauve, pink, burgundy, reddish-brown||almandine garnet, amethyst, apatite, dolomite, feldspar, fluorite, halite, lepidolite, rhodonite, rose quartz, ruby|
|black, brown-black, dark gray, gray||biotite mica, diopside, fluororichterite, hornblende, titanite|
Other words to describe the intensity of colors:
- dark, very dark, light, pale
- dull, shiny
Words that describe how color is distributed:
- splotchy, mottled, speckled
- layered, banded
Words used to describe the way light reflects off of the surface of a mineral:
|dull / earthy||very dull, mainly in minerals that are porous||kaolinite, orthoclase|
|waxy||like the surface of a candle||opal, chalcedony|
|greasy / oily||nepheline|
|pearly||like a pearl, play of colors, light||talc, muscovite mica|
|silky||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”|
|resinous||looks like freshly-broken shellac, usually yellow-brown||sphalerite|
|adamantine||high luster, almost brilliant, “diamond-like”||sphalerite|
|sub-metallic||silvery or metallic luster but mineral is transparent or translucent when in small slivers||hematite|
|metallic||very shiny, like processed metals, highly reflective, opaque minerals||pyrite, gold, silver|
Other words that describe luster:
- 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.
Because 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.
|very light||< 2||borax|
|light||2 – 2.5||gypsum, halite, selenite, ulexite|
|average||2 – 3||calcite, dolomite, feldspar, muscovite mica, quartz, talc, turquoise,|
|above average / slightly heavy||3 – 4||biotite mica|
|heavy||4 – 5||almandine garnet, apatite, barite, celestite, chalcopyrite, fluorite|
|very heavy||5 – 10||galena, hematite, magnetite, nickel-iron, pyrite|
|extremely heavy even for a metallic mineral||> 10||gold, silver|
|super heavy||20+||must be platinum!|
Gemstones are sold by weight. A chart that relates to the specific gravity of gemstones can be found at:http://www.gemselect.com:80/calibrated-size/calibrated-size-chart.php 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 shapes are:
- acicular / radiating needles ~ crystals that grow in fine needles
- blebs ~ rounded blobs
- botryoidal ~ looks like the top of the bunch of grapes
- concretion ~ 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
- fibrous ~ looks like fibers, threads, parallel lines
- geode ~ spherical, round shape that is hollow inside, often lined with crystalshexagonal prism with pyramid termination ~ hexagonal cross-section, with pointy ends (terminations)
- hexagonal prism with rounded ends ~ 6 sided cross-section, with rounded ends
- hexagonal pyramid ~ sharp 6 sided pyramid, often seen in clusters
- mamillary ~ rounded like botryoidal but a bit bigger than a bunch of grapes
- massive ~ 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 known as a “book”
- termination ~ the end of a complete crystal
Cleavage is when a mineral breaks with smooth flat surfaces. Cleavage can be described as perfect, good, imperfect, poor. 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 regularly
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 smooth
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
- friable ~ crumbles easily
- malleable ~ 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 Friedrich Mohs Scratching tools:
- fingernail (2.2)
- copper penny (3.5)
- pocket knife or common nail (5.2)
- a 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?
|1 Very Soft||Easily crumbles. Can be scratched with a fingernail (2.2)||Talc|
|2 Soft||Can be scratched with a fingernail (2.2)||Gypsum, Soapstone|
|3 Soft||Can be scratched with a copper penny (3.5)||Calcite|
|4 Semi-Hard||Can be scratched with a common nail (5.2)||Fluorite|
|5 Hard||Can be scratched with a common nail (5.2).||Apatite|
|6 Hard||N. B. Mineral of hardness 6 or more will scratch glass.||Feldspar|
|7 Very Hard||Can be scratched with a concrete nail (7.5).||Quartz|
|8 Very Hard||Topaz|
|9 Extremely Hard||Used in industrial tools for cutting, grinding & sanding.||Corundum|
|10 The Hardest||Diamond is used to cut all minerals including diamonds.||Diamond|
For a more detailed article about HARDNESS, click here. A good site that discusses hardness in gems and minerals can be found at http://www.gemselect.com/gem-info/gem-hardness-info.php
TRANSPARENCY ~ Transmitting Light Through Minerals
A mineral can be:
|celestite, quartz (rock crystal),selenite|
|calcite, quartz, sphalerite|
GLOSSARY of Words & Phrases
- acicular / radiating needles ~ crystals that grow in fine needles
- adamantine ~ very shiny like a gemmy crystal, almost brilliant
- botryoidal ~ looks like top of bunch of grapes
- chatoyant ~ shines like a cat’s eye because of fibers
- chemicals ~ 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 rock, 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 & space to grow properly
- crystals ~ minerals that form slowly have a distinctive crystal shape
- cubic ~ 6 equal, square faces
- dendritic ~ branching, tree-like growths
- dog-tooth ~ shaped like the canine tooth, like a dog’s tooth
- dull / earthy ~ very dull, mainly in minerals that are porous
- earth’s Crust ~ 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
- gemstones ~ rocks & minerals that have been cut & polished, used for decoration and are usually rare and valuable
- 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 &pressure due 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 break regularly
- 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 & study of minerals
- rockhound ~ a lover of rocks, minerals & fossils who collects specimens in the field
- rocks ~ non-living matter, made of 2 or more minerals
- rock cycle ~ rocks are constantly forming, wearing down and forming again, very slowly however
- 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 fossils
- specific gravity ~ how heavy something feels when compared to what you would expect, heft, weight, mass, density
- 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 or 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 – or Tests on the Go 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:
- Habit – 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. quartz.
- 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 field use.
- Opacity – 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 or sapphire!
- 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” size.
- 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 a string.
- Oddballs – Fluorescence – of some use (in the dark) for fluorite, some calcite, scheelite and sphalerite .. Oh yes! And diamonds too!
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