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Hardness – Specimens 1 to 9 are listed in order of the Mohs hardness scale. Try an experiment. Scratch one
mineral with another and then do it in reverse order. You will find that the lower numbers are softer than the
higher numbers. Go ahead. Scratch some minerals. That is what they are here for. There are some common
materials that you can also use for hardness testing: A fingernail is hardness of 2 ½, a copper penny (before
1983) is 3, a steel knife is 5, glass is about 5 ½, and a steel file 6 ½.
Note: not every mineral is present in every box. We run out and have to substitute other minerals. Check the
name and not the number.
Safety tips:
- If you are scratching glass, put the glass plate on the table, don’t hold it in your hand. Use thick
pieces of glass (like a Ward’s scratch plate or a pickle bottle), not thin things like glass slides.
- When doing a hardness test with a nail, be careful not to stick yourself or anyone else.
- Don’t put any mineral in your mouth.
- Always wash your hands after handling minerals.
1. TALC, hardness of 1 Hydrated Magnesium Silicate. This mineral is usually found in metamorphosed oceanic crust. It
is so soft that your fingernail will scratch it. Ground talc is often used as “baby powder.”
2. GYPSUM, hardness of 2 Hydrated Calcium Sulfate: A very common mineral that precipitates as seawater evaporates.
The most common use is for making sheet rock wallboard.
3. CALCITE, hardness of 3 Calcium Carbonate: A very common mineral that typically is precipitated by organisms like
clams, which use it to make their skeletons. When large amounts of these biological products
accumulate and get cemented together, the rock is called limestone. Calcite may be dissolved
and re-precipitated by hot ground water as large and beautiful crystals.
4. FLUORITE, hardness of 4 Calcium Fluoride: Typically found in beautiful crystals in hydrothermal deposits (200 degrees C
and 5000 PSI) along with metallic ore minerals. Ancient peoples discovered that fluorite
(“flowing stone”) would make many ore minerals melt more easily. This mineral shows 4
directions of cleavage.
5. APATITE, hardness of 5 Calcium Phosphate with some fluorine: Apatite comes from both organic and inorganic
sources. Many living creatures (like sharks) make their skeletons and teeth out of hydrated
calcium phosphate. Their accumulated remains are mined for fertilizer. Inorganic apatite is
found in igneous rocks.
6. MICROCLINE, hardness of 6 Potassium Aluminum Silicate: Microcline is one of a family of minerals called feldspars. They
are found in igneous rocks and weather to clay minerals. Note that there are two directions of
cleavage at nearly right angles (micro-cline = little angle) to each other.
7. QUARTZ, hardness 7 Silicon dioxide: Quartz is the most common mineral on the continents of the earth. Sand is
primarily quartz that has eroded from somewhere else, like granites, and has been concentrated
by rivers. When minerals cement the sand together, the rock is called sandstone. Quartz can be
recognized by its hardness (it will scratch glass) and its lack of cleavage. Broken quartz looks
just like broken glass, and is just as sharp. It grows in hexagonal crystals with striations on the
sides. Many common forms of quartz (agate, chalcedony, flint, chert, jasper) are made of tiny
crystals, the result of rapid crystallization.
8. TOPAZ, hardness 8 - When nicely colored (blue, yellow, pink) and transparent, topaz is a gemstone.
Aluminum fluohydroxysilicate: Topaz is found in pegmatite veins associated with granites. A
pegmatite is the last material to crystallize as a large igneous body cools. Pegmatites
frequently contain rare and beautiful minerals like topaz. Our specimens are rounded, waterworn pebbles collected from a stream in Brazil, so they do not show crystal faces. Topaz has a
single direction of weakness, called a cleavage plane, which shows up if a pebble is broken.
9. CORUNDUM, hardness 9 –
Aluminum oxide: Corundum is found in silica-deficient, high temperature metamorphic rocks or
in aluminum-rich igneous rocks. If silica is present, corundum will not form because aluminum
silicates are much more stable. The diagnostic test for corundum is its hardness. You will find it
harder than anything in this box, harder than any mineral but diamond. Minor impurities can
color corundum red (ruby) or blue (sapphire).
Manganese silicate: Rhodonite (hardness 5 ½ to 6) is usually found in metamorphic rocks where
the heat and pressure create an environment in which the manganese dissolves out of low
concentration sources like shale. It can form large masses of beautiful pink stone. You might
mistake this mineral for the other common pink mineral, rhodochrosite (hardness 3 ½ to 4),
except that rhodonite is much harder. Try a hardness test and see. Rhodonite is a manganese
ore and is cut as a semi-precious gemstone.
Potassium aluminum silicate: Muscovite is found in igneous and metamorphic rocks. The
distinguishing characteristic of this mineral is thin, flexible sheets, which can be split off. It has
a hardness of 2 to 2½.
Iron carbonate: Siderite can be found in sedimentary rocks where it precipitates from seawater
as a cement. It also is found in hydrothermal ore veins where it forms in masses with good
cleavage like the sample you have. This mineral is harder than calcite (hardness 3) and softer
than fluorite (hardness 4). Try it.
13. CALCITE crystals on matrix.
Calcium carbonate: Look at your specimen for broken tips on the ends of the crystals. You will
see 3 directions of cleavage. Test the hardness to see that this mineral is harder than gypsum
and softer than fluorite.
14. AZURITE, blue from Morenci copper mine in Arizona. The green is malachite.
Copper carbonate: Azurite and malachite are common copper minerals. A hardness test is
difficult because the azurite is a fluffy mass of crystals, which crush when you try a hardness
test. Both azurite and malachite are important copper ores.
15. CELESTINE from Lampasas County, Texas. It occurs in cavities in limestone.
Strontium sulfate: The red color in fireworks is from strontium. A distinguishing characteristic
for this mineral is that it is heavy. You can do a specific gravity test to find that this mineral is 4
times the density of water, but first just try a “heft test”. Take a piece of celestine in one hand
and a similar sized piece of calcite or quartz (density of 2.9) in the other and jiggle them.
Compare it with siderite.
16. QUARTZ, variety amethyst - from Rio Grande do Sul, Brazil - from cavities in basalt.
Silicon dioxide: The purple color is from iron. If you were to wrap this crystal in 10 layers of
aluminum foil and heat it for 2 hours at 400 degrees F, it would change color to yellow as the
iron changes oxidation state.
17. GYPSUM, “roses” from the Chihuahuan desert of northern Mexico.
Calcium sulfate: Gypsum grows in many crystal shapes. This kind grows in sand that is
saturated with “hard water” and the calcium sulfate precipitates at the water evaporates. The
white deposits on your showerhead are mostly gypsum. You will see that the hardness is the
same no matter what shape the crystals are in.
18. PYRITE, known as "fool's gold", from the silver mines of Peru.
Iron sulfide: diagnostic properties are its brassy gold color, its brittleness and, when present,
its cubic crystals. Once a minor iron ore, most pyrite is mined today for its trace content of
precious metals like gold and silver.
19. STILBITE occurs in cavities in basalt - from Siberia, Russia, Nova Scotia, Canada and India.
Hydrous sodium calcium aluminosilicate. These crystals grow rapidly. Artificial zeolites are
used in ion-exchange applications, such as water purifiers.
20. CHRYSOCOLLA is a hydrous copper silicate. From an Arizona copper mine. The blue-green color
and the botryoidal growth (lots of tiny, radiating crystals that make a bubbly-looking mass) are
typical. Chrysocolla’s nice color and luster make it a semi-precious gemstone.
21. ARAGONITE - twinned crystal from Clarendon, Texas.
Calcium carbonate, like calcite, but orthorhombic instead of hexagonal. Unless you have
crystals, it is almost impossible to tell calcite from aragonite. Some organisms, like oysters,
make aragonite hard parts.
22. GARNET crystal from Malawi in Africa.
Aluminosilicate group with extremely variable cation substitution: Garnets occur in many rock
types. Though only slightly harder than quartz, garnets are pretty tough. Because they break
with a conchoidal fracture (like glass), making lots of little sharp pieces, crushed garnets are
used to make sandpaper. When clear and a pretty color (red, orange, green) garnet is faceted
as a gemstone.
23. WAVELLITE, green sprays on gray Big Fork chert. From Arkansas.
Hydrated aluminum phosphate: a secondary mineral (formed by breaking down phosphates or
Al-rich metamorphic rocks). Its bright green, lustrous, radiating, fibrous crystals make it a
sought-after specimen but it has little economic importance.
24. VANADINITE, tiny orange to red crystals from Arizona.
Lead chloro-vanadinate: a secondary mineral formed by oxidizing lead ores in deserts. (Wash
your hands after handling this mineral, even though it is not very soluble in water.)
25. BERYL, small six-sided (hexagonal) crystals from New Hampshire.
Beryllium aluminum silicate: Beryl is really hard (H=8), right up there with topaz. Transparent,
colorful members of the beryl family (green – emerald, blue – aquamarine, pink – morganite) are
prized as gemstones. Beryl is the only important source of the element beryllium.
26. ZOISITE, pink with epidote (green), quartz (white to gray), Baja California.
Hydrous calcium alumino-silicate: forms in pegmatites and metamorphic rocks that form when
high Ca rocks are subjected to considerable pressure. Crystals radiate from central points,
indicating rapid growth.
27. ADAMITE on brown goethite, Mapimi, Durango, Mexico.
Zinc arsenate: a secondary mineral formed by oxidizing zinc sulfides and other primary ore
minerals. A minor ore of zinc. The yellow varieties are fluorescent and phosphorescent. (Wash
your hands after handling this mineral, even though it is not very soluble in water.)
28. KYANITE in QUARTZ from Brazil – Blue blades in a milky quartz matrix.
Aluminum silicate: formed from clay that is metamorphosed at high temperatures and
pressures. Besides its blue color, the most diagnostic feature is that it is harder when
scratched across to the crystals than parallel to them. A knife will make a scratch along the
crystals but not across them.
29. HEMIMORPHITE, white to gray crystals in brown to black goethite, Mexico.
Hydrous zinc silicate: Naturally colorless, hemimorphite is often colored by minor impurities.
Even though it is a silicate, it is pretty soft, H=4.5 to 5.
30. BARITE SAND ROSE or nodule, from Norman, Oklahoma.
Barium sulfate: Barite is a white mineral. The red sandstone in which they are found colors
these “roses”. A small amount of barium gets “stuck” to the clay minerals that make up a
marine mud deposit. When diagenesis transforms that mud into shale, some of the barium is
expelled into the ground water. That ground water will be forced through a permeable
sandstone layer on its way to the ocean and the barium will encounter sulfate from seawater
and be precipitated as barite. Try a “heft” test and note that barite (density of 4.3) has a much
greater specific gravity than quartz (density of 2.9) or calcite.
Other minerals you may encounter.
26. SPHALERITE, dark brown to black, bright cleavage and dull crystal faces, with yellow chalcopyrite.
Zinc sulfide: Sphalerite is the primary ore of zinc. It has good cleavage and a substantial heft.
28. Calcite – cut pieces of calcite onyx, Mexico.
Calcium carbonate: This is another form of calcite that is deposited by dripping water in a cave.
29. SMITHSONITE, pinkish bubbly crusts, Mexico.
Zinc carbonate: Minor amounts of cobalt color this specimen pink.