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Rocks and Minerals of Salt Lake County


We want you to recognize 5 of the 3000 or so minerals.

Click on a mineral to learn more:

quartz >feldspar > mica >calcite >little dark minerals


What will scratch what?

Hardness is a measure of how easily the surface of a mineral can be scratched. The harder a mineral is to scratch, the higher its hardness number. The most widely used hardness scale was developed by Friedrich Mohs in the early 1800s and is known as the "Mohs Scale of Hardness". Most minerals have a relatively narrow range of hardness and can be put into the scale. Human-made materials such as a knife blade or pane of glass can be ranked on the scale. So the game becomes "What can scratch what?".

Mohs Scale of Hardness
Number Mineral Scratched by
1 Talc

finger nail to 2.5
copper penny 2.5-3

2 Gypsum
3 Calcite knife, nail or rock hammer
up to 5.5
4 Fluorite
5 Apatite
6 Feldspar a pane of window glass
5.5 to 6.5
7 Quartz a steel file is 6 to 7
8 Topaz  
9 Corundum  
10 Diamond diamond

(Mica, with a hardness of 2 to 2.5, is one of our 5 minerals but not on the hardness scale)

You’ve been handed a rock with some crystals and you wonder whether the crystals are quartz or calcite. If a knife blade (or rock hammer) can leave a scratch-mark, you conclude the crystals are quartz or calcite?