Conceived in 1812 by German mineralogist, Friedrich Mohs, Mohs' scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material.  It is one of several definitions of hardness in mineral science.  Mohs based his scale on ten minerals that are all readily available except the last one, diamond.  The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material.  For example, if some material is scratched by apatite but not by fluorite, its hardness on Mohs scale is 4.5.  On Mohs scale a fingernail has hardness 2.5, a copper penny about 3.5, a knife blade 5.5, window glass 5.5, a steel file 6.5.  Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. 

1.  Can be scratched easily with a fingernail.  Talc

2.  Can be scratched with a fingernail.  Gypsum

2.5 - 3.  Gold, Silver

3.  Can be scratched with a coin.  Calcite

4.  Can be scratched easily with a knife.  Cannot scratch glass.  Flourite

4 - 4.5.  Platinum

4-5.  Iron

5.  Can be scratched with a knife.  Can just scratch glass.  Apatite

6.  Can be scratched with a steel file.  Easily scratches window or bottle glass.  Orthoclase (such as Moonstone)

6.5.  Iron Pyrite (often called "fool's gold")

7.  Easily scratches metal, glass and softer stones.  Quartz (such as Amethyst, Carnelian, Chalcedony and Citrine)

7 - 7.5.  Garnet

7.5 - 8.  Beryl (such as Emerald and Aquamarine)

7 - 8.  Hardened steel

8.  Scratches quartz and softer stones.  Topaz

9.  Scratches topaz and softer stones.  Corundum (such as Ruby and Sapphire)

10. Scratches ruby.  Diamond