Mohs scale

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A scale for classifying minerals based on relative hardness, determined by the ability of harder minerals to scratch softer ones. The scale includes the following minerals, in order from softest to hardest: 1. talc; 2. gypsum; 3. calcite; 4. fluorite; 5. apatite; 6. orthoclase; 7. quartz; 8. topaz; 9. corundum; 10. diamond.

Wiktionary

1. n. A scale of the hardness of minerals based on their ability to scratch one another