In 1822 German mineralogist Friedrich Mohs (1773-1839) established a scale for measuring the relative hardness of stones that is still in use today (note that hardness has nothing to do with the fragility of a stone, i.e. its resistance to knocks.) Lapidaries must pay attention to a specific characteristic of hardness, resistance to abrasion, as this determines how they adjust their wheels. Friedrich Mohs’s scale, which bears his name, is based on the relative hardness of minerals which he measured in terms of their resistance to scratching with a pointed object. The Mohs scale is divided into ten grades, with no specific ratio between each one, as follows: (1) talc (2) gypsum (3) calcite (4) fluorspar (5) apatite (6) orthoclase (7) quartz (8) topaz (9) corundum (10) diamond.