Mohs scale of mineral hardness
Diamond is always at the top of the scale, being the hardest mineral.  There are ten minerals in Mohs scale, talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and for last and hardest, diamond.  Because the Mohs scale was made long ago, it is not exactly correct - for example, several minerals are now known to be harder than the diamond.   The Mohs scale may not be perfect, but field geologists still find it very useful.