How hard is it? A simple guide to Mohs’ Scale

Gemstones and mineral specimens have many properties.  One that is particularly important for jewelry is hardness.  Why? A stone that is soft should not be used in an unprotected setting or in a piece that is likely to be bumped.

For example, I had a lovely opal ring with four opal cabochons in prong settings.  Not realizing at first how fragile opal can be, I wore the ring regularly.  It didn’t take long before I’d chipped one of the stones.  I was slightly more careful with it but still managed to chip a second stone and lose that first one (as it was loose in its setting).  Fortunately, we were able to match up the opal, cut and polish new cabochons and restore the ring completely.

For stones that are below 7 on the Mohs’ hardness scale, dust can do some damage if rubbed into the stone as quartz particles are fairly common in dust.  To help identify how hard a stone is, there is a “simple hardness tester” that can be used as a guide and to provide an idea of what the various scale values mean along with an example in parenthesis[1]:

  • 1 (talc) and 2 (gypsum): can be scratched with a fingernail
  • 3 (calcite): can be scratched with copper coin
  • 4 (fluorite): easily scratched with knife
  • 5 (apatite): can be scratched with knife
  • 6 (orthoclase): can be scratched with steel file
  • 7 (quartz): scratches window glass
  • 8 (topaz), 9 (corundum), 10 (diamond) : no simple test 

I would not recommend actually trying any of the scratch tests on finished stones but rather use the information to help understand what the various Mohs values represent.  Note that the scale is relative, so that the difference between a 4 and 5 on the scale is not the same as the difference between 9 and 10.

[1] Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p20 (based on the table “Relative and Absolute Hardness Scale”)


Gemstones from outer space

With the recent excitement over asteroids and the meteor impact in Russia, it reminded me that some of the gemstones and minerals we enjoy on Earth have their origins in outer space.

While tektite forms from Earth material, it is believed that tektites are formed only as a result of extraterrestrial impacts.  Tektite resembles volcanic glass but has a significantly lower water content.  These bumpy rocks often appear in a drop shape although some people have faceted tektite like a gemstone.  Moldavite is a name given to a greenish form of tektite.

According to an article from the National Science Foundation, natural black diamonds formed in outer space.  “The presence of hydrogen in the carbonado diamonds indicates an origin in a hydrogen-rich interstellar space.”[1]  This also seems to provide an explanation as to why black diamonds are not found in mines and why it is so unusual to find good quality natural black diamonds over 1 carat in size (think fragments of objects hitting the Earth).   Note that many black diamonds in jewelry stores will have fine print to indicate that the stones were irradiated or radiation treated (for color) – those stones are most likely not the same chemical composition as natural black diamonds.

Peridot, also called olivine, has not only been found in meteorites but it has also been found on Mars[2].  This green gemstone is also formed deep inside the Earth and can be expelled during volcanic eruptions.   I will never look at peridot the same!

[1]  “Diamonds from Outer Space: Geologists Discover Origin of Earth’s Mysterious Black Diamonds”

[2] “Gemstones found on Mars”


An example of peridot jewelry from our site at

Peridot set in Sterling Silver

A diamond rainbow…of treatments

People are most familiar with the white (colorless) diamond. However, over the past year, I have seen major chain jewelry stores adding jewelry with a variety of colored diamonds in their catalogs.

Natural diamonds can be found in yellow, orange, brown/cognac, and black (commonly) as well as the less common pink, light green, and lavender. Deep blue, red and dark green are extremely rare.

If you don’t mind irradiated stones, you may find a less-expensive stone that has been heated and irradiated to produce fluorescent yellows, electric blues, magenta, coral pinks, chocolate browns, blacks and greens. Many of the black diamonds on the market (including that in the aforementioned jewelry catalogs) have been irradiated or heated under very high temperatures. Read the fine print!

That’s not to say that white diamonds haven’t been treated in some way. A laser can be used to vaporize black inclusion in a white diamond. This is a permanent treatment which, like all treatments, should be disclosed by a seller.

Fractures and cracks may have been filled – a process which is not always permanent. This, like the laser treatment, would be visible under magnification.

High temperature and pressure may also be used on yellow-tinted diamonds to make them white.

If you are purchasing from a reputable seller, any treatments should be disclosed. Think twice about a “bargain” diamond if you don’t have a certificate from a respected laboratory or if you cannot have it verified yourself.