What is the scale called to measure hardness of gemstones?

The Mohs scale. # Talc # Gypsum # Calcite # Fluorite # Apatite # Feldspar # Quartz # Topaz # Corundum # Diamond The Mohs scale, however, is an purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is almost four times as hard as corundum. Check the related link for more info.