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|Mohs' Hardness Scale|
Crude but Practical
In 1822, Friedrich Mohs, a German mineralogist devised a crude but practical method of comparing hardness or scratch resistance of minerals. It has become universally known as Mohs scale or Moh's scale as the grammatically challenged would call it, actually it should properly be called Mohs' scale.
Comparative not Scalar
It should more accurately be called a table, because it is not to scale, that is the numbers allocated to different minerals are not proportional to their actual scratch resistance, so that the scale is really an ordered list.
Starting From Scratch
Moh took ten well known, easily available minerals, and arranged them in order of their "scratch hardness".
If a specimen to be tested can be scratched by a known mineral from the list, it is softer than that mineral. If it in turn will scratch another known mineral, it is harder than that mineral. This gives a very quick and easy field test for hardness. As such is it very useful for mineralogists. It is too destructive to be commonly used in gemmology, but is available, and can be valuable on rough gemstones.
Don't Try This At Home
A crude form gets used on shop windows by people testing out their diamond rings, in the mistaken belief that only diamond will scratch glass.
Measures of Hardness
There are many different aspects of materials which could be considered as a measure of hardness. Hardness can mean resistance to scratching, indentation, bending, breaking, abrasion, cleavage, or fracture. It is easy to confuse durability or toughness with hardness.
A very simple example is to consider a glass ball and a rubber ball. Glass is harder than rubber, but rubber is more durable. Try bouncing both on a hard floor, the glass ball will shatter, whereas the rubber ball will bounce.
The aspect of hardness which is measured by Mohs' test is the scratchability of a mineral.
Other scales of hardness include Brinell's Vicker's, Knoop, Meyer, Rockwell, and a rebound (dynamic or absolute hardness) test measured with a sclerometer.
The hardness of each mineral relative to the others varies according to which test is performed, and also hardness can vary according to the grain direction, or crystallographic orientation, of the specimen. We have given typical Brinell hardness figures for each of the minerals in the table. Diamond's hardness cannot be measured on the Brinell scale, because a diamond indenter is used for the test itself, but it is many times harder than corundum.
Sapphire and ruby are the well known varieties of corundum.
Amethyst, citrine, rock crystal, and cairngorm are all varieties of quartz.
Importance for Gemstones
Generally, high scratch resistance is desirable for gemstones, and a Mohs' hardness of 7 or higher is important. The principal reason is that a common cause of abrasion is sand, which is silica grit (quartz), and is commonly present in dust. Stones which are softer than quartz are not suitable for everyday use as facetted jewellery gemstones, particularly in rings, although many are beautiful and attractive.
Some gems, such as pearls, coral, turquoise, lapis lazuli, amber, and opal are quite soft, but are usually polished into cabochons or beads, rather than facetted, and therefore do not show scratches so easily. All these gemstones have been successfully used in jewellery for many centuries.
You may wish to visit some of our other pages:-
Harder than Diamond
It surprises most people to learn that things exist which are harder than diamond.
Mohs Scale or Moh's Scale?
We originally titled this page Moh's Scale, but have since added Mohs Scale to the title. Although this is incorrect and ungrammatical, and would make Lynne Truss, the author of bestselling book Eats, Shoots and Leaves, groan. It would seem however that the days of accurate users of punctuation are now sadly numbered, particularly in regard to apostrophes. Instant research, consisting of typing "Moh's Scale" into Google returned 12,300 results, wheareas searching for "Mohs Scale" produced 91,000 results, so outvoted by a ratio of 7.4:1, we gave in, but not without a small protest. These are web pages which have been authored by people who, presumably profess some degree of expert knowledge on the subject of Mohs; Scale. This raises two questions in our mind. It they don't know how to spell it correctly, what would the results be for non-experts, the people doing the searching? If these experts can't even spell the title of their subject matter accurately, what chance is there of getting the actual informational content correct? But then again, they probably simply copied it from our website; if so, how come they missed the apostrophe? Alright, we know, that makes three questions!
For those who don't know the difference, the term Moh's Scale refers to a scale owned or invented by Moh, Mohs' Scale refers to a scale owned or invented by Mohs, whereas a Mohs Scale, if such a thing existed, would be a system for comparing one Moh with another, probably by some aspect of physical difference. As we can't ever remember seeing even one Moh (we never met the guy), never mind two or more of them, we can't really see much point in the existence of a scale to compare them. There is a song alleging that one man went there.
When we created this page in about 1998, the author was under the impression that the originator of the scale was called Friedrich Moh. In September 2005, we noticed that his name was in fact Friedrich Mohs. Whoops!
Interestingly, we notice many other "experts" have added a Mohs hardness scale page to their jewellery or gemstone websites, and duplicated our error in ascribing the scale to "Moh". They say imitation is the sincerest form of flattery!
Of course, this gives us little room to criticise those cited above, as we should all refer to "Mohs' Scale"!!!
Obituary / Biography
In a belated attempt to pacify the soul of the long departed Herr Mohs, we append below a brief obituary, culled from Wikipedia:
Friedrich Mohs (January 29, 1773 - September 29, 1839) was a German geologist/mineralogist.
Mohs, born in Gernrode, Germany, studied chemistry, mathematics and physics at the University of Halle and also studied at the Mining Academy in Freiberg, Saxony. After assuming the position of a foreman at a mine in 1801, Mohs moved in 1802 to Austria, where he was employed in trying to identify the minerals in a private collection of a Banker. As part of this task, he started classifying minerals by their physical characteristics, in spite of their chemical composition, as had been was done traditionally.
This emphasis on physical characteristics was at odds with the prevailing chemical systematics. Minerals are now classified by chemical characteristics, but the physical properties are still useful in field examination.
In 1812, Mohs became professor in Graz; in 1818, professor in Freiberg, Saxony; in 1826, professor in Vienna. He created a hardness scale that is still used as Mohs scale of mineral hardness.
Mohs died during a trip to Italy in Agordo near Belluno.
We have seen a photograph of Mohs' memorial headstone which is in the Vienna Central Cemetery. It clearly shows his name as Mohs.
A to Z of Gemstones
Hardness of Different Gold Alloys
Desirable Gemstone Attributes & Characteristics
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