Mohs' scale of hardness was developed by Friedrich Mohs and
measures the hardness of "rock" on a scale of 1 to 10 using
a scratch test. ie 2.5 can be scratched by a fingernail:
1=talc, 2=gypsum, 3=calcite, 4=flourite, 5=apatite,
6=orthoclase, 7=quartz, 8=topaz, 9=corundum, 10=diamond.
However, please keep in mind that each project is more like granite than a single element - hardness varies from song to song.