The hardness of a mineral is determined by what it will scratch and what scratches it.  In 1822, the Austrian mineralogist Moh devised a scale from
1 - 10 using 10 common minerals:
 
1 - Talc 6 - Orthoclase
2 - Gypsum 7 - Quartz
3 - Calcite 8 - Topaz
4 - Fluorite 9 - Corundum
5 - Apatite 10 - Diamond
We do not always have this set of minerals but the following are usually readily available:  
Finger Nail
2.5
Copper Coin
3.5
Pocket Knife
5.5
Scratches Glass
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