Have you ever wondered why a diamond can scratch almost anything, while a piece of calcite can be scratched with a simple copper coin? The answer lies in a mineral's hardness—one of the most important properties used to identify minerals.
What Is Mohs Hardness?
The Mohs Hardness Scale measures a mineral's resistance to scratching. Developed in 1812 by German geologist Friedrich Mohs, the scale ranks minerals from 1 (softest) to 10 (hardest).
It's important to remember that the Mohs Scale isn't a straight line. For example, a diamond (10) is far more resistant to scratching than corundum (9), even though they're only one number apart.
The Mohs Hardness Scale
- 1 – Talc: The softest mineral, easily scratched with a fingernail.
- 2 – Gypsum: Soft enough to scratch with a fingernail.
- 3 – Calcite: Can be scratched by a copper coin.
- 4 – Fluorite: Scratched by a steel nail.
- 5 – Apatite: Similar hardness to window glass.
- 6 – Orthoclase Feldspar: Scratches glass with ease.
- 7 – Quartz: One of the most common minerals on Earth and hard enough to scratch steel.
- 8 – Topaz: A durable gemstone prized for its beauty.
- 9 – Corundum: Includes sapphires and rubies.
- 10 – Diamond: The hardest naturally occurring mineral on Earth.
Why Does Hardness Matter?
Hardness helps geologists identify minerals in the field and tells us how durable a mineral will be in everyday use.
For example:
- Quartz is commonly used in jewelry because it's resistant to scratches.
- Softer minerals like calcite require extra care to prevent damage.
- Gemstone hardness is one reason some stones are better suited for rings, while others are ideal for pendants or display pieces.
Try It Yourself
You don't need a laboratory to begin testing mineral hardness. Many common household items have known hardness values:
- Fingernail: about 2.5
- Copper coin: about 3
- Steel nail: about 4.5
- Glass: about 5.5
By carefully testing which materials can scratch a specimen—and which cannot—you can narrow down its identity.
(Always test an inconspicuous area and never scratch valuable mineral specimens.)
Every Mineral Tells a Story
Hardness is just one of many characteristics that make minerals so fascinating. Along with colour, crystal shape, streak, and luster, it helps reveal the identity and history of each specimen.
The next time you pick up a crystal or mineral, remember that its hardness isn't just a number—it's part of the incredible story written by nature over millions of years.
Whether you're just starting your collection or have been collecting for years, understanding the Mohs Hardness Scale will help you appreciate the remarkable diversity of the mineral world.