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Diamond Hardness

In 1822 a German minerologist Freidrich Mohs chose 10 well known and easily procurable minerals and arranged them in order of their scratch hardness. In decending order:


  • Diamond
  • Sapphire
  • Topaz
  • Quartz
  • Orthoclase Feldspar
  • Apatite
  • Fluorspar
  • Calcite
  • Gypsum
  • Talc

    The list is not quantitative and represents an order only. The gap between Diamond and Sapphire is far greater than the gap between Sapphire and Talc.

    Most minerals exhibit a difference in hardness in different crystal directions. Diamond is no exception although the variations in hardness are not very great, they are of high practical importance. Diamond is harder than any other substance on earth, and to grind and polish diamond the only possible abrasive is diamond itself. This only works because one can abrade diamond along its softer directions by the action of its harder directions. It was found by diamond cutters that the direction parallel to the crystal axes are those of least hardness.

    Scratch hardness can now be measured more scientifically with an instrument called a sclerometer, named from the Greek meaning hard. A diamond point is moved across the flat surface of a material at greater and greater pressure until a scratch is produced. Diamond is given a value of 140,000 with Corundum (Sapphire) at just 1,000. There is also a Knoop hardness scale which gives diamond, on the cube surface, a value of 10,400 kg per square millimetre.

    Knoop
    Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests Knoop Principleare mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests. The magnifications required to measure Knoop indents dictate a highly polished test surface. To achieve this surface, the samples are normally mounted and metallurgically polished, therefore Knoop is almost always a destructive test.

    This hardness in diamond is important in its attractiveness as a gem and in its cutting efficiency as an industrial mineral. Resistance to abrasion is important as a scratch will impair the transparency, lustre and brilliancy of the stone. The hardness enables diamond to be highly polished to achieve its adamantine lustre and the facet edges to be sharp and remain so.
     
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