The Mohs Hardness Scale is a qualitative ordinal scale developed by German mineralogist Friedrich Mohs in 1812. It ranks minerals from 1 (softest, talc) to 10 (hardest, diamond) based on their ability to scratch one another. A mineral with a higher Mohs number can scratch any mineral with a lower number. It remains the most widely used hardness test in geology and mineralogy due to its simplicity.

A scratch test involves taking a material of known hardness and attempting to scratch an unknown material with it. If the known material leaves a visible scratch or groove on the unknown material, the known material is harder. If no scratch appears, the unknown material is harder. This simple principle allows geologists and hobbyists to identify minerals in the field without specialized equipment.

No, it is not linear. The Mohs scale is an ordinal scale — the gaps between values are not equal. For example, diamond (10) is approximately 4 times harder than corundum (9) in absolute hardness (measured by indentation tests like Vickers or Knoop). Corundum (9) is about 2 times harder than topaz (8). The scale only indicates relative ranking, not proportional differences.

Diamond is the hardest known natural material, ranking 10 on the Mohs scale. It is composed of pure carbon atoms arranged in a tetrahedral crystal lattice, making it incredibly resistant to scratching. Only another diamond can scratch a diamond. However, diamond is brittle — it can be shattered with a hammer blow despite its extreme hardness. Synthetic materials like aggregated diamond nanorods and cubic boron nitride can be even harder.

You can perform basic hardness tests using common household items:
• Fingernail — Mohs ~2.5 (scratches talc and gypsum)
• Copper penny/coin — Mohs ~3.5 (scratches calcite)
• Iron nail — Mohs ~4.5 (scratches fluorite)
• Steel knife blade — Mohs ~5.5–6 (scratches apatite and orthoclase)
• Glass bottle — Mohs ~5.5
• Unglazed porcelain tile — Mohs ~7 (useful for streak tests)
These make excellent field-expedient testing tools for mineral identification.

Generally, no — materials of equal hardness typically cannot scratch each other. When two minerals have the same Mohs hardness, they may produce some surface wear or powdering under pressure, but a clear scratch is unlikely. This is why diamond (10) can only be scratched by another diamond, and even then, it requires specific crystallographic orientation.

Mohs hardness is a qualitative scratch test — it only determines relative ranking (1–10). Vickers hardness is a quantitative indentation test that measures the size of an indent left by a diamond pyramid under a known load. Vickers provides a continuous numerical value (in kgf/mm² or GPa) and is much more precise. For example, talc has a Vickers hardness of ~2.4 MPa, while diamond has ~10,000 MPa — a difference of over 4,000 times!

Friedrich Mohs chose 10 common minerals that were readily available in the early 19th century and spanned the full range of known hardness at the time. Diamond was the hardest known natural substance. While some modern synthetic materials exceed diamond's hardness (like wurtzite boron nitride or aggregated diamond nanorods), the original 1–10 scale remains the standard because it is practical, memorable, and sufficient for identifying virtually all natural minerals.

Jewelers use the Mohs scale to assess gemstone durability. Stones with hardness below 7 (like opal at ~5.5–6.5 or pearl at ~2.5–4.5) are more susceptible to scratches from airborne quartz dust (hardness 7) and require more careful setting and maintenance. Sapphires and rubies (corundum, 9) and diamonds (10) are highly prized for engagement rings precisely because they resist daily wear. This is why diamond is the most popular choice for wedding jewelry.

Glass has a Mohs hardness of approximately 5.5. Any mineral with a hardness of 6 or above can scratch glass. This includes orthoclase (6), quartz (7), topaz (8), corundum (9), and diamond (10). A simple steel knife blade (~5.5–6) may leave faint scratches on glass. If you find a mineral that easily scratches glass, it is likely quartz or harder — a useful field identification clue!