Mohs Scale of Mineral Hardness

Friedrich Mohs, a German mineralogist, was given the job of cataloging Europe’s most renowned mineral collections. Acquired in 1802 by a wealthy Viennese banker, the collections were the subject of Mohs’ study and classification according to mineral parameters such as color, taste, smell, magnetism, specific gravity, brittleness, and hardness. The last parameter, hardness, was a puzzlement. Mohs couldn’t figure out how to quantify hardness other than to describe how hard a mineral was compared to another.

Mohs started with the hardest and softest minerals available at the time. It was 1812 at the time. Diamonds, or pure carbon, were the hardest materials around while talc, or magnesium silicate, was the softest. All other minerals were somewhere in between.

Mohs chose available materials to exemplify each hardness on a comparative, non-linear scale. The chart below shows the minerals he chose. Because absolute hardness could not be defined, Mohs used a relative scale with diamond at 10 (maximum hardness) and talc at 1 (minimum hardness). Other minerals’ hardness was determined by whether another mineral could scratch it. Diamond will scratch mostly anything it’s so hard. But other minerals aren’t so lucky. Here’s how they stack up. A mineral cannot scratch the mineral listed above it.

Mohs hardness/Mineral
10/diamond
9/corundum
8/emerald
7/quartz
6/feldspar
5/apatite
4/fluorite
3/calcite
2/gypsum
1/talc

It’s important to note that increments on the Mohs scale are not equal. For example, diamonds at 10 on the Mohs scale are actually four times harder than corundum (a mineral group that includes rubies and sapphires) at 9 on the Mohs scale.

Other materials besides minerals can have a Mohs hardness measure. Metals of gold, silver, and copper rank less than 3 on the Mohs scale; platinum and iron at about 4. Human body parts such as tooth enamel rank at about 5 and fingernails at 2.2. Organic materials such as pearls, jet, and amber rank at about 5. A knife blade at 5.5 will scratch any of those organic materials. So will picking your teeth with diamonds! Seriously, the point to remember is that the Mohs scale was developed by human beings scratching one substance with another and ranking them accordingly in a relative order.

So, why does knowing the Mohs hardness of a material matter? Five important reasons include scratching, identification, abrasives, cleaning, and erosion. Let me explain.

Scratching
Have you noticed how gemstones are wrapped in paper and plastic when buying at gem shows? Do you wonder why jewelry boxes are lined with velvet? Gems, jewelry, and metals can scratch one another according to their Mohs scale hardness. Keeping them separated reduces the risk of scratching one another when put in the same place.

Identification
Mohs scale hardness helps to identify minerals. Hardness is unique for each mineral and independent of other obscuring factors such as color, form, dirt, and treatments.

Abrasives
Abrasive materials are used to clean, polish, sand, smooth, and otherwise abrade some other material to some degree. When the Mohs hardness is known, the abrasive capability can be put to work on softer materials. For example, garnet’s Mohs hardness is 7.5. Garnet sandpaper can produce a fine finish on hardwood surfaces.

Cleaning
Cleaning is a gentler form of abrasion. Cleaning always removes some molecules from the surface. The harder the material, the easier it is to remove surface dirt without removing the valuable gemstone material beneath. That’s why, for example, ultrasonic cleaning is so good for the hardest gemstone materials such as diamonds, rubies, sapphires, and emeralds. Softer materials in the ultrasonic cleaner will be giving up the material itself, or suffer damage or changes to the surface.

Erosion
When touring this magnificent world of ours, notice the variety of geological formations formed by erosion. Soft minerals erode faster than hard ones, leaving voids where they used to be with harder minerals remaining as capstones, monuments, outcrops, and overhangs.

Start your own list of materials for Mohs hardness. You will learn about your most valued materials, keep them, clean them, observe them, wear them, and care for them with their individuality in mind!

Sources: personal experience

Victoria Finlay, Jewels, A Secret History, Ballantine Books, a Division of Random House, New York, 2006

http://www.3m.com

http://www.amfed.org/t_mohs.htm