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The Mohs Hardness Scale Explained: 10 Minerals From Softest to Hardest

From talc to diamond, explore the 10 levels of the Mohs Hardness Scale.

1 β€” Talc

Easily scratched with a fingernail. The softest reference mineral on the Mohs scale, with a slippery feel β€” ground finely, it's used in baby powder and cosmetics.

2 β€” Gypsum

Just barely scratched by a fingernail. About as hard as a fingernail itself, gypsum hardens when mixed with water, making it widely used in drywall, cement, and plaster casts.

3 β€” Calcite

Scratched by a copper coin. The main component of marble and limestone, easily identified by the way it fizzes when an acid like vinegar is applied.

4 β€” Fluorite

Easily scratched by a knife blade. Known for glowing under ultraviolet light β€” the word "fluorescence" itself derives from this mineral's name.

5 β€” Apatite

Just barely scratched by a knife blade. Chemically very similar to the main components of human tooth enamel and bone. Also mined as a source of phosphate for fertilizer.

6 β€” Orthoclase

Not scratched by a steel knife blade. Belongs to the feldspar group, the most common mineral group in Earth's crust. A major component of granite, also ground for use in ceramic glazes.

7 β€” Quartz

Easily scratches glass. The most common mineral in Earth's crust after feldspar, and the main component of sand. Its piezoelectric property (generating electricity under pressure) makes it useful in watches and electronic quartz oscillators.

8 β€” Topaz

Can scratch quartz. Prized as a gemstone for its clear, transparent crystals, and one of the birthstones for November. Pure crystals are colorless, but trace impurities give it various colors.

9 β€” Corundum

Can scratch topaz. The hardest natural mineral after diamond. When red, it's called ruby; any other color, sapphire β€” they are in fact the same mineral.

10 β€” Diamond

The hardest substance found in nature, capable of scratching every other mineral. Made purely of carbon atoms, it sits at the top of the Mohs scale. Its exceptional hardness makes it valuable not just as a gem but also in industrial cutting tools.

What Is the Mohs Hardness Scale?

The Mohs Hardness Scale was devised in 1812 by German mineralogist Friedrich Mohs. It ranks minerals' relative hardness on a 10-point scale β€” from 1 (softest) to 10 (hardest) β€” based on which of two minerals scratches the other when rubbed together.

Does a Higher Number Mean Proportionally Harder?

The Mohs scale shows relative order, not an absolute measure of hardness difference. In reality, diamond (10) is far, far harder than corundum (9) β€” the gap is much larger than the numbers alone suggest, so hardness differences can't be judged by the numbers alone.

Frequently Asked Questions

How hard are a fingernail, a coin, and glass?

As general reference points: a fingernail is about 2.5, a copper coin about 3.5, and glass or a steel blade about 5.5-6.5 β€” handy for roughly estimating a mineral's hardness in the field with simple tools.

Is it true that ruby and sapphire are the same mineral?

Yes, that's correct. Both gems are the mineral corundum β€” trace amounts of chromium give it a red color (ruby), while other trace elements produce other colors (sapphire).