Hardness | Emoche ᛜ Gemstones & Jewelry

Don’t be fooled by everyday uses of the word “hard.” In the world of gemstones, hardness has a specific meaning. It’s all about how well a crystal resists scratches.

Imagine a fingernail easily scratching glass. In everyday terms, glass is hard. But for a gemologist, glass is relatively soft. Many materials considered hard in daily life can scratch gemstones.

Why is this so? It boils down to the atomic structure of the crystal. Strong bonds between atoms make it difficult to separate layers on the surface. A harder material can break these bonds, leaving a scratch.

The Mohs Scale: Ranking Crystal Toughness

To measure this “scratchability,” gemologists use the Mohs Scale. Developed in 1812, this system ranks ten common minerals based on hardness. Here’s the scale, with harder materials listed below:

Diamond (10)
Corundum (9)
Topaz (8)
Quartz (7)
Feldspar (6)
Apatite (5)
Fluorite (4)
Calcite (3)
Gypsum (2)
Talc (1)

The scale shows how scratches work. Each mineral can scratch those below it but gets scratched by those above. Minerals with the same hardness, like a ruby and sapphire, can’t scratch each other.

It’s interesting to note that the Mohs Scale isn’t perfectly linear. Hardness jumps significantly at higher numbers. Corundum is twice as hard as topaz, and diamond is four times harder than corundum! In fact, only diamonds can scratch other diamonds.

Precision and Challenges in Hardness Measurement

Gemstone hardness can vary slightly depending on its makeup and structure. Measuring it precisely can be tricky. Sometimes, what looks like a scratch might just be a mark left by a supposedly harder material.

Scientists use fractional scores (like 5.5) when necessary. But remember, the exact number isn’t as important as the general hardness range. This article covers the entire range you’ll find in gemology literature.

One exception is kyanite, a mineral with surprising hardness variations within a single crystal. But for most gemstones, hardness falls within a narrow range, typically differing by just one unit on the Mohs Scale.