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What is the Absolute Hardness of Diamond?

One of the factors that is considered important regarding minerals is their hardness which means how well they resist the scratch of material with known hardness. Thus hardness is a diagnostic property in minerals that is used in the field identification of minerals. In material science, hardness is defined as resistance to deformation in response to mechanical stress, indentation, and abrasion. Among natural substances, diamond due to its unique compact carbon structure is considered the strongest known material. 

Moh’s Scale of Hardness

There are different definitions of strength and in terms of minerals ability of one material to scratch the other. Most hardness is the measure of the relative hardness and the resistance to scratching between them. One of the most followed strength scales is devised by German mineralogist Friedrich Mohs on the basis of one mineral to scratch another mineral. In 1812 he selected 10 common minerals i.e., diamond, corundum, topaz, quartz, feldspar, calcite, gypsum, and talc and arranged them in order of relative hardness. Each of these minerals represents things of similar hardness. He arranged minerals in such a way that hardness ranges on the scale of 1 to 10; talc is easiest to scratch with a score of 1 and diamond being the hardest of all with the score of 10 as mentioned in the table below


According to his scale diamond was among the top of all the minerals in terms of hardness and strength. The curve of the Mohs scale of hardness shows exponential growth that shows that diamond’s absolute hardness around 1500 to 1600 times harder than talc. This is why it is so much easier to scratch talc as compared to the diamond.  In terms of their absolute hardness diamond is 4 times the absolute hardness of corundum. 


Various things are used for scratching, for initial testing human fingernail is used that has the hardness of 2.5 according to the Mohs scale i.e., it will be able to scratch talc and gypsum but not calcite.  Some others are penny, glass plate, steel file, and streak plate, each having different hardness values. 

This is the easiest hardness test as all you have to do is to scratch it on some other surface and all the ten minerals used in this scale are easily available. As this scale measures the relative hardness of minerals that makes it not absolute so it is not a linear scale, so it is considered too arbitrary and not precise enough

Rosiwal Absolute Hardness

It is more preferable to Mohs sale as it determines the absolute hardness of a mineral instead of relative hardness that makes it very useful in the research field. It is expressed as the abrasion resistance that is measured at the laboratory. This absolute scale of hardness starts with the base value of corundum i.e., 1000, and depicts the unmatchable hardness of diamond i.e., 140,000. 


Knoop Absolute Hardness 

This scale is also used to measure the absolute hardness of a mineral calculated by measuring the indentation produced when the diamond is pressed on the surface of the tip. This hardness test also depicts the absolute hardness test of diamond around 7000 i.e., maximum as compared to other minerals. 


Other common tests/ instruments used for depicting the absolute hardness of diamonds are as follows

Absolute hardness is measured by a Sclerometer which is an instrument that uses a diamond pyramid to measure the absolute hardness, through a scratch with a diamond pyramid. The non-linearity of the absolute hardness is very clear from this graph as diamond is not just 10 times hard than talc but is mighty 1600 times harder. 

Another measuring scale is the Micro indentation test that is often preferred. Indentation hardness is the quantitative measurement of hardness by applying mechanical stress on a substance with known characteristics. It uses an indenter pressed on the surface of the tested material using a monoploid force of about 1 to 1000 kg. By comparing different minerals, diamonds was among the top in terms of absolute hardness as it can withstand maximum pressure

Similarly, streak plate mineral test is used to identify other diversified properties of minerals and is more visible. The maximum hardness that can be assessed using this method is a hardness of 6.5 means anything with hardness above that can’t be tested with the streak test. 

Hydraulic press test is used to determine the strength and hardness of diamond by using hydraulic press pressure. In this test, you have to place different minerals in between the hydraulic press to check how much pressure in kilograms that mineral or metal can withstand. This test also depicts the absolute hardness of diamond was among the top of all the minerals.

What gives Diamonds this Absolute Hardness

The most famous example for demonstrating the variability of hardness in an element is carbon. It can be in the form of the diamond, the strongest known material and so soft in the form of graphite that we can crush easily. So the most important aspect that is responsible for the hardness of any mineral is its chemical bonding between atoms. Hardness is mainly controlled by the strength of the bonding between atoms and partially by the composition and size of atoms.

The molecular structure of a diamond is composed of five carbon atoms sharing their electrons in a tetrahedral lattice. The bonds between these atoms are very strong and very hard to break. The hardness of a mineral relates to its weaker spot. Covalent bonds of graphite are as strong as diamond but weak Vander wall forces make graphite so soft with the hardness of 1.5 whereas diamond has a hardness of 10 and its covalent bonds are extremely strong. Hardness is a vector property as many minerals show varying degrees of hardness depending on the hardness in which they are crushed like the hardness of kyanite varies from 4.5 to 7 depending upon the direction from which it is tested. 

Since a lot of energy is required to break the bond so this means that diamond has very high melting and boiling points. The strong covalent bond within diamond atoms depicts that they don’t have free electrons around making them terrible electric conductors but excellent heat conductors. Diamond is 5 times better heat conduction ability than copper. Due to excellent thermal conductivity diamonds can be used in electronic parts. In addition, when diamonds are heated at around 752 degrees Fahrenheit they experience thermionic emission in which the intense heat causes diamonds to shed their electrons. For this reason, there is a suggestion that diamonds can be used in solar cells in future

Uses of Diamonds’ Absolute Hardness 

Diamonds are pretty hard that only diamonds can scratch other diamonds. Such strong covalent structures are valued in biological applications as well as electronics, optics, industry, research, etc. Their hardness is useful in many cases 

  • Because of strong covalent bonding, diamonds can be used in industrial applications grinding, drilling, cutting, and polishing. Diamonds also cut faster and are more durable than other materials like steel. 
  • It can be placed on the tip of cutting tools as it helps in cutting strong metals and minerals. Its effect of absolute hardness is beneficial in cutting cores in oil wells and mineral exploration
  • Diamonds are most the thermally conductive material of any solid i.e., they make a great heat sink for electronics.  Due to this property, they are used to create electronic circuits and microchips. 
  • Additionally, they experience thermionic emission when heated to around 400°C; the added heat energy allows the diamond to emit electrons. This property can be used in solar panels to collect extra electrons and increase the efficiency of solar energy collection 

Tips to Determine Hardness 

In practical mineralogy, few good practices must be followed to determine the hardness of the mineral. Some are discussed below

  • There is a difference between scratch and mark. Sometimes when one mineral is softer than the other, portions of the first will leave a mark that may be mistaken for a scratch. Such a mark can be rubbed off whereas the true scratch will be permanent 
  • Always use fresh specimens. The surfaces of some minerals are frequently altered to other materials that are much softer than the original material. So it is recommended to test hardness, a fresh surface of the specimen must be used.

The absolute hardness of hardness is determined by different scales like Mohs scale of hardness, Knoop scale, Rosiwal scale for absolute hardness depicts it remains at the top of all minerals in terms of absolute hardness. Thus diamond is the strongest known material on the surface of the earth. The reason for its amazing absolute hardness lies in the covalent bonding between its carbon atoms. Such absolute hardness makes it a wonder mineral that is more than just a piece of crystallized carbon; because of its absolute hardness, it has so much potential in both science and technology. 

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