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Identifying Rocks & Minerals

First, examine your specimen with a magnifying glass and take note of its outside appearance. We recommend acquiring either Rocks, Minerals & Gemstones or A Field Guide to North American Rocks & Minerals to help identify whether your find is a 'mineral' or a 'rock'. Use a Mineral Testing Kit. Your kit should include a streak plate (for color test), glass plate (for hardness test), plastic dropper bottle (for acid test), a magnet and 5/10X magnifier.

Once you have identified your sample you can proceed accordingly.If you have identified your specimen as a 'mineral' you should look for the mineral's transparency. If you can see through the specimen, it is transparent. If light can pass through, but the specimen cannot be seen through, your mineral is translucent. Minerals that do not let light through are called opaque.

Determining Hardness

Original Mohs Scale of Mineral Hardness

The relative hardness of minerals is determined according to Mohs Scale, named after the German mineralogist, Friedrich Mohs (1773-1839), who devised it in 1812. In the original Mohs Scale, ten minerals were arranged in order of increasing hardness and were assigned the numbers one to ten. These ten minerals are shown in the first column of the table below:

Hardness	Mineral	Associations and Uses
1	Talc (Softest)	Talcum powder. (can be scratched by a fingernail.)
2	Gypsum	Plaster of paris. Gypsum is formed when seawater evaporates from the Earth's surface. (May be scratched by a fingernail or by a copper coin.)
3	Calcite	Limestone and most shells contain calcite. (Can be scratched by a steel pocket knife or sometimes a copper coin. Will scratch a fingernail, may scratch a copper coin.)
4	Fluorite	Fluorine in fluorite prevents tooth decay. (Can be scratched by a steel pocket knife. Will scratch a fingernail and a copper coin.)
5	Apatite	When you are hungry you have a big "appetite". (Can be scratched by a steel pocket knife. Will scratch a fingernail and a copper coin.)
6	Orthoclase	Orthoclase is a feldspar, and in German, "feld" means "field". (Will not scratch glass but will scratch steel blades, copper coins & fingernails.)
7	Quartz	(Will scratch glass, steel blades, copper coins & fingernails.)
8	Topaz	The November birthstone. Emerald and aquamarine are varieties of beryl with a hardness of 8. (Will scratch glass, steel blades, copper coins & fingernails.)
9	Corundum	Sapphire and ruby are varieties of corundum. Twice as hard as topaz. (Will scratch glass, steel blades, copper coins & fingernails.)
10	Diamond (hardest)	Used in jewelry and cutting tools. Four times as hard as corundum. (Will scratch all of the above.)

A substance with a higher Mohs number is capable of scratching a substance with a lower number.
Mohs selected these ten minerals because they were common or readily available. The scale is not a linear one, and is somewhat arbitrary. For example, Fluorite at four is not twice as hard as Gypsum at two; nor is the difference between Calcite and Fluorite similar to the difference between Corundum and Diamond.

Hardness is used in a rough way to inform mineral identification in the field. Real minerals out in the field can look remarkably alike. This may be due to weathering, variations in their chemical structure from the ideal, or clathrate inclusions that simply change the colour of the mineral. Sometimes faulting and metamorphism can induce facets and planes in a mineral that aren't natural to it, so that the mineral looks like another.

The Extended Mohs Scale

Mohs Substance	Hardness
Liquid	1
Substance as indicated in the standard scale	2-6
Vitreous pure silica	7
Quartz	8
Topaz	9
Garnet	10
Fused zirconia	11
Fused alumina	12
Silicon carbide	13
Boron carbide	14
Diamond	15

Some common field tests:

2.5	Fingernail (will scratch 1-2 hardness)
2.5-3	Gold, Silver
3	Copper penny
4-4.5	Platinum
4.5	Iron
5.5	Knife blade
6-7	Glass
6.5	Iron pyrite
7+	Hardened steel file

On each level of the scale a mineral can be scratched by something of the same or higher level, but nothing lower.

Match the hardness table above with the items listed below it. Test your mineral specimen by trying to scratch it with your fingernail. If it doesn't scratch, next try a copper penny. If you are able to scratch your specimen with the penny but not with your fingernail, it has a hardness between 2.5 and 3.5. If the specimen does not scratch with a penny, try a knife blade or glass. A diamond can only be scratched by another diamond.

One last test that is commonly used is called a streak test.
A mineral's "streak," or color when it is finely powdered, is always the same, even when the color of the mineral varies. (The color of the streak can be very different from the color of the mineral itself.) Rub your specimen across a piece of porcelain tile (a "streak plate") and examine the color it leaves behind. Once you have performed your tests, compare your results with a Rocks and Minerals field guide to come to a final identification of your specimen.

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