Hardness -Mohs Scale of Ten Minerals

HARDNESS- the term in context of gems and precious stones is meant to imply the ability to resist scratching

Harness of a stone does not imply toughness .  Hard stones may still be brittle and crack, break or shatter if struck

Hardness of a stone distinguishes value based on enduring and resisting wear.  In other words hardness represents a stones resistence to abrasion and scratching.  A gem would not make fine jewels if its beauty could be easily tarnished.

A method called the Mohs scale has been the common way to determine hardness of a stone since 1812.

To test the hardness of precious stones that have not been cut or polished, the following scale of ten minerals was devised by Frederich Mohs, a German mineralogist.  Below is the scale as presented in an 1890 publication.

No. 1. Talc. Very soft; is easily broken or scratched with the finger-nail. 
No. 2. Rock-salt. Soft ; scratched with difficulty with finger-nail ; readily cut with a knife.
No. 3. Calcite. Low degree of hardness; not to be scratched with finger-nail; easily scratched with a knife.
No. 4. Fluor spar. Fairly hard ; is slightly scratched by a knife, but easily attacked with a file.
No.5. Apatite. Medium hardness; does not scratch glass, or only faintly ; does not give out sparks against steel ; easily attacked with a file. 
No. 6. Felspar. Easily scratches glass; is attacked by a file, and gives some sparks against steel. 
No. 7. Quartz. Quite hard; is only slightly attacked by file; gives sparks readily against steel.
No. 8. Topaz. Very hard; is not attacked by a file.
No. 9. Sapphire. Hardest of all minerals but the diamond; attacks all other minerals.
No. 10. Diamond. Attacks all minerals is not attacked by any.


How to determine the hardness of stones from Handbook of Precious Stones -1890

“To find the hardness of a stone, begin to test with the softest mineral, so that when the number is reached which will scratch the stone, there has been no injury to the specimen under examination. “
“Half numbers are determined by the ease or difficulty with which a stone is scratched.  For example, a stone which will resist No. 7 (quartz) and which is only faintly attacked by No. 8 (topaz) may be safely put down as 7.5.  While a stone which resisted No. 7 and yielded easily to No. 8 is to be classed as 7 in hardness. “
“These tests are readily applied to crystals or unpolished gems.” 
“With the polished stone greater care must be observed, and while a file test is often satisfactory, there is always the danger of striking the cleavage and breaking off a small piece of the stone. “