GIA GEMOLOGIST ARTICLE

The Mohs Scale of Hardness

A commonly used scale in the trade, it is often misconstrued or misunderstood.This article explains all!

Antony Zagoritis

GIA Graduate Gemologist
ICA Ambassador to Kenya


  • What is the Mohs Scale ? :  The Mohs scale was developed by German mineralogist Friedrich Mohs and is a scale used by gemologists to classify a gem mineral’s resistance to scratching or in other words “the difficulty or ease with which one mineral can be scratched by another”.
  • The scale assigns each mineral a value between 10 (being the hardest and most resistant to scratching) and 1 (the least resistant to scratching).
    Mohs Scale Hardness.jpg
  • Mohs Scale Ranking Chart      
  • Diamond is the only mineral with a value of 10, being the hardest mineral. As the graph above shows, there is a large drop from 10 to 9 where the next hardest minerals are categorized (Ruby & Sapphire). However, the graph is not linear, Diamond is actually 3 ½ times harder than Ruby and Sapphire, which in turn are twice as hard as Topaz. 

  • Hardness vs Toughness
  • A common misconception made in both the trade and by layman is equating “hardness” which is "the resistance to scratching" to a mineral’s susceptibility to damage as a whole. Minerals are susceptible to damage in different ways - they could break, deform, or crumble instead of scratching. Hard materials often break when subjected to stress. This is a lack of toughness and is assessed using a different scale. Emerald, for example, has a hardness of 8 but its propensity to contain numerous inclusions and fractures can mean that is not very durable in a setting, even though its resistance to scratching is relatively good.

  • General Listing of Gemstones by Mohs Scale Value
  • The table below gives a good breakdown of various minerals and their values on the Mohs scale.

  • List of Gemstones by Mohs.jpg
ABOUT THE AUTHOR Antony Zagoritis, Bsc, GG

Antony completed his GG (Graduate Gemologist) in 1998 at the Gemological Institute of America in California after a degree a Business at the University of Bath, in England. He has extensive experience in the colored gemstone trade with over 20 years buying rough at the source. He is currently the Ambassador to Kenya for the ICA (International Colored Gemstone Association) in New York which is the worldwide body for colored gemstones.

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