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Hardness Testing
Diamond
The Hardest Known Sunstance
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 Diamond is the hardest naturally occurring substance known; it is also the most popular gemstone. Because of their extreme hardness, diamonds have a number of important industrial applications.
The hardness, brilliance, and sparkle of diamonds make them unsurpassed as gems. In the symbolism of gemstones, the diamond represents steadfast love and is the birthstone for April. Diamonds are weighed in carats (1 carat = 200 milligrams) and in points (1 point = 0.01 carat). In addition to gemstones, several varieties of industrial diamonds occur, and synthetic diamonds have been produced on a commercial scale since 1960.
A very high refractive power gives the diamond its extraordinary brilliance. A properly cut diamond will return a greater amount of light to the eye of the observer than will a gem of lesser refractive power and will thus appear more brilliant. This high dispersion gives diamonds their fire, caused by the separation of white light into the colors of the spectrum as it passes through the stone.
The scratch hardness of diamond is assigned the value of 10 on the Mohs scale of hardness; corundum, the mineral next to diamond in hardness, is rated as 9. Actually, diamond is very much harder than corundum; if the Mohs scale were linear, diamond's value would be about 42.
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Several items have been provided for your use in determining physical properties, including a glass plate (H=5.5), an unglazed porcelain streak plate (H=7.5), and a nail or pocket knife (H=5.0). Try scratching each of these with the others to determine their relative hardnesses. Where does your fingernail fit into this hardness scale? A copper penny? The hardness of any object is controlled by the strength of bonds between atoms and is measured by the ease or difficulty with which it can be scratched. Diamond is the hardest mineral, because it can scratch all others. Talc is one of the softest; nearly every other mineral can scratch it. We measure a mineral's hardness by comparing it to the hardnesses of a standardized set of minerals first established by Friederich Mohs in the early nineteenth century, or with the common testing materials that have been calibrated to those standards.
The Mohs Hardness Scale is a relative scale. This means that a mineral will scratch any substance lower on the scale and will be scratched by any substance with a higher number. Diamond is not 10 times harder than talc or 1.1 times harder than corundum, as would be the case with an absolute hardness scale. Most often we are able only to narrow down hardness to within a certain range; for example, if an unknown mineral scratches a copper penny but does not scratch a glass plate, its hardness must be greater than 3.0 and less than 5.5. Usually this range of values is sufficient to identify an unknown. Note: please always use care when testing hardness on a glass plate. If the glass gets broken DO NOT handle it!
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