I usually refer to the Moh’s scale of mineral hardness in my descriptions of jewelry pieces.
Friedrich Mohs’ a German mineralogist, created this scale in 1812. This scale helps determine how soft or hard a gemstone is. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. He based the scale on 10 readily available minerals.
To help you understand, on the Mohs scale, a pencil “lead” or graphite has a hardness of 1; A fingernail 2.5; a copper penny, about 3.5; a knife blade, 5.5; window glass, 5.5; and a steel file, 6.5.
Diamond is the hardest. To cut diamond, you actually cut it against diamond.