Hardness, in mineralogy, is a measure of 'scratchability'; a material of a given hardness will scratch a material that is not as hard. More than a century ago, mineralogist Friedrich Mohs devised a hardness scale for minerals, called the Mohs scale. A mineral of hardness 7, such as quartz, will easily scratch a softer mineral, such as calcite (hardness 3). In this photo, the calcite specimen is a banded 'onyx'. On the Mohs scale, talc is number 1 and diamond is number 10.