BACKGROUND

Hardness is the resistance of a substance to being scratched by another material. A scratch is a permanent line or groove cut into the surface of the material being tested and caution should be taken not to damage a valuable gem.  The test should be made in an inconspicuous place such as the girdle of a cut stone.  Don't attempt to test gems until you have acquired a great amount of practical experience testing common minerals.  Rough gem material is more applicable for testing than cut stones, which could potentially receive serious damage by overzealous testing.

 One important property of a gem is that it must be hard enough to resist everyday wear and tear. Harder gems take a better polish and thus appear more lustrous. If a gem is softer than 7 on Moh's Scale, it will lose its luster as dust from the air (dust contains silica, hardness 7) slowly makes minute scratches on the gem's surface.
 
 

While a number of hardness scales exist, the most commonly used in geology and gemology is the Moh's scale; Moh's assigned relative numerical  values to pure minerals as follows:
 
 
 

Friedrich Moh, a German mineralogist, proposed his scale in 1822. Because it uses universally available common minerals, Moh's scale became well accepted.  It is only a relative scale since the minerals are not equally times harder than each other.  For example, the difference between diamond's hardness and that of corundum is greater than the difference in hardness between all other minerals on the scale (1 talc-9 corundum)!

All minerals and natural substances have a value somewhere between 1 and 10.  A useful mnemonic, a device for remembering the mineral names in the correct sequence, 1 - 10, is the sentence "The Good Cat Finds An Old Quilt To Curl Down."

Several common objects have a hardness identical (or very closely matching) those on the scale and are available for use just about everywhere. These include:
 
 
 

Most minerals have a hardness between 2 and 7, using the common materials to determine the hardness usually works . In practice, use the softest test object on the sample first.  If it is harder, it will scratch the substance leaving a permanent mark that will not wipe away; items of equal hardness should scratch each other. Results are best when using relatively pure samples.  Impurities could give a false reading; so inspect the sample and the tested object with a magnifier. 

SAFETY PRECAUTIONS

 The following minerals occur in granite.  Which mineral in granite is most resistant to abrasion (scratching)?