<P> The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly . The samples of matter used by Mohs are all different minerals . Minerals are pure substances found in nature . Rocks are made up of one or more minerals . As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale . The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, 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 the Mohs scale would fall between 4 and 5 . "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye . Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number . While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number . </P> <P> The Mohs scale is a purely ordinal scale . For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum . The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples . </P> <Table> <Tr> <Th> Mohs hardness </Th> <Th> Mineral </Th> <Th> Chemical formula </Th> <Th> Absolute hardness </Th> <Th> Image </Th> </Tr> <Tr> <Td> </Td> <Td> Talc </Td> <Td> Mg Si O (OH) </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> Gypsum </Td> <Td> CaSO 2H O </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> Calcite </Td> <Td> CaCO </Td> <Td> 9 </Td> <Td> </Td> </Tr> <Tr> <Td> </Td> <Td> Fluorite </Td> <Td> CaF </Td> <Td> 21 </Td> <Td> </Td> </Tr> <Tr> <Td> 5 </Td> <Td> Apatite </Td> <Td> Ca (PO) (OH, Cl, F) </Td> <Td> 48 </Td> <Td> </Td> </Tr> <Tr> <Td> 6 </Td> <Td> Orthoclase feldspar </Td> <Td> KAlSi O </Td> <Td> 72 </Td> <Td> </Td> </Tr> <Tr> <Td> 7 </Td> <Td> Quartz </Td> <Td> SiO </Td> <Td> 100 </Td> <Td> </Td> </Tr> <Tr> <Td> 8 </Td> <Td> Topaz </Td> <Td> Al SiO (OH, F) </Td> <Td> 200 </Td> <Td> </Td> </Tr> <Tr> <Td> 9 </Td> <Td> Corundum </Td> <Td> Al O </Td> <Td> 400 </Td> <Td> </Td> </Tr> <Tr> <Td> 10 </Td> <Td> Diamond </Td> <Td> </Td> <Td> 1500 </Td> <Td> </Td> </Tr> </Table> <Tr> <Th> Mohs hardness </Th> <Th> Mineral </Th> <Th> Chemical formula </Th> <Th> Absolute hardness </Th> <Th> Image </Th> </Tr>

According to the mohs hardness scale what is the hardness of quartz