Developed by German mineralogist and geologist Friedrich Moh in 1812, it's a scale used to measure resistance of minerals to scratching. It's an internationally recognized standard to rank hardness from a scale of 1 to 10, 1 being the softest to 10 being the hardest (how the surface of a gem responses when in contact with a sharp point). This is not to be confused with a gem’s toughness, which is defined by how well a gem can survive an impact or resist breaking, chipping or cracking. (For instance, a diamond is 10 on the scale, but can still break from say an impact of a hammer).
Why is this information useful? The higher on the scale a gem is, the more durable it is, meaning the more suitable it is to daily wear and tear of jewelry.
Here's a reference of how some gems stand on the Mohs Scale of Hardness:
|Amber||2 - 2.5|
|Pearl||2.5 - 3.5|
|Fluorite , Malachite||4|
|Turquoise, Glass, Lapis Lazuli||5.5 - 6|
|Peridot||6.5 - 7|
|Quartz (Amethyst, Citrine, Smoky Quartz), Jadeite||7|
|Zircon, Tourmaline, Garnet, Agate (7 - 7.5)||7.5|
|Emerald||7.5 - 8|
|Chrysoberyl, Cubic Zirconia||8.5|
|Corundum (Ruby, Sapphire)||9|
And here's a reference based on everyday items:
So, although there is a wide range of stones which are used in jewelry for instance Turquoise, Amber, Opal, which are lower on the Mohs scale, it just means extra care has to be taken when worn.