What physical property can be shown by scratching one material with another?
As the article relates, the relative hardness of minerals had been considered since antiquity. Moh arbitrarily placed talc at no. 1 on the scale, i.e. the least hard mineral, and at 9, and 10, the hardest minerals, carborundum, and diamond. The scale is non-linear, but probably highly useful for a geologist.