Mohs Scale.
Also known as: Mohs Hardness Scale
A 1 to 10 scale of mineral hardness based on which materials can scratch which, used to compare and identify gemstones.
The Mohs scale is the small ruler sitting behind every "is this stone scratch-resistant?" question. It was proposed in 1812 by the German mineralogist Friedrich Mohs, who picked ten reference minerals and ranked them by which could scratch which. Talc sits at 1 (a fingernail will mark it), gypsum at 2, calcite at 3, fluorite at 4, apatite at 5, orthoclase feldspar at 6, quartz at 7, topaz at 8, corundum (ruby and sapphire) at 9, and diamond at 10.
The scale is comparative, not linear. The jump from 9 to 10 is far larger than the jump from 1 to 2. A useful rule of thumb: anything below 5 will be scratched by everyday grit, since street dust contains a lot of quartz at 7. This is why softer stones like selenite, malachite, and turquoise need careful handling, while quartz and harder stones can live in a pocket without much fuss.
For buyers, hardness is a practical guide more than a quality marker. A soft stone is not a lesser stone. It just asks for different care.