Geological classification

What is the name of the hardest mineral?

The Mohs scale (mineralogical hardness scale) is a set of reference minerals for determining relative hardness by scratching. 10 minerals, arranged in order of increasing hardness, were taken as standards. Consists of 10 hardness standards: talc – 1; gypsum – 2; calcite – 3; fluorite – 4; apatite – 5; orthoclase – 6; quartz – 7; topaz – 8; corundum – 9; diamond – 10. Minerals with an index below 7 are considered soft, while those above 7 are considered hard. In general, the bulk of natural compounds have a hardness of 2 to 6. The hardness scale was proposed in 1811 by the German mineralogist Friedrich Mohs. The hardness of a stone is the resistance that its surface offers when you try to scratch it with another stone or other object; hardness is a measure of the cohesiveness of the atomic structure of a substance. The hardness of the same stone can be different in different directions. Kyanite stands out among other minerals due to the large difference in hardness in different directions: its hardness varies from 5 to 7, and in some directions the sample can be scratched with a knife, but not in others. The values ​​on the scale from 1 to 10 correspond to 10 common minerals, from talc to diamond. A mineral’s hardness is measured by finding the hardest reference mineral it can scratch; and/or the softest reference mineral that scratches the given mineral. Everything is very simple. For example, if a mineral is scratched by apatite but not by fluorite, then its hardness is in the range of 4 to 5. Intermediate degrees of stone hardness are expressed as fractions. Thus, the number 8 1/2, referring to chrysoberyl, means that it scratches topaz in much the same way as it scratches itself with corundum. Pyrope garnet is slightly harder than quartz (7) and slightly softer than beryl (7 1/2), so its hardness is designated as 7 1/4. It is important to remember that cryptocrystalline, finely porous and powdery varieties of minerals have false low hardnesses. For example, hematite in crystals has a hardness of 6, but in the form of red ocher it is less than 4. Every owner of a diamond ring knows that a diamond easily scratches window glass. Diamonds are used to cut glass. If we try other precious stones, we will find that they also scratch glass, but not so easily, and then topaz scratches window glass, but the topaz itself is scratched by corundum, which in turn gives in to the all-powerful and hardest diamond. Samples of the same minerals obtained from different places differ from each other in the complexity of the cutting and polishing process. It is said of diamonds from Kalimantan and New South Wales that they are considerably harder than those from South Africa and other places and that they are difficult to cut. Ceylon sapphires are harder than rubies, while Kashmir sapphires are softer. The Mohs scale is intended for a rough comparative assessment of the hardness of materials according to the softer-harder system. The material being tested either scratches the standard and its hardness on the Mohs scale is higher, or it scratches the standard and its hardness is lower than the standard. Thus, the Mohs scale only informs about the relative hardness of minerals. In addition to the Mohs scale, there are, of course, other methods for determining the hardness of minerals, but the different hardness scales cannot be unambiguously correlated with each other. Several more accurate systems for measuring the hardness of materials have been adopted in practice, none of which covers the entire spectrum of the Mohs scale.

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Household hardness measuring instruments

  • the nail leaves a scratch on plaster and softer substances;
  • ordinary window glass is slightly softer than feldspar;
  • The steel blade of a knife is slightly harder than feldspar, approaching quartz in hardness, and scratches glass easily.

Classification of natural stone materials by hardness

Solid
Medium hard
Soft
Granite, gneiss, diorite, syenite, gabbro, labradorite, teschenite, diabase, quartz porphyry, basalt

Marble (achromatic and chromatic), conglomerate, breccia, limestone, sandstone, volcanic tuff, calcareous tuff, shale

Linear hardness

Linear hardness is determined by the absolute hardness scale, not the Mohs scale. Here is the absolute hardness scale:

Talc – 1 – Can be scraped off with a fingernail
Plaster – 3 – Scratchable with a fingernail
Calcite – 9 – Scratched by a copper coin
Fluorite – 21 – Easily scratched with a knife
Apatite – 48 – Can hardly be scratched with a knife
Orthoclase – 72 – Scratchable with a file
Quartz – 100 – Scratches window glass
Topaz – 200 – Easily scratches quartz
Corundum – 400 – Easily scratches topaz
Diamond – 1600 – Can’t be scratched by anything (and at the same time it easily scratches corundum)

Diamond is a mineral that is outstanding in every way. Like an unsightly doll (a nugget diamond really does not look like anything special in appearance), after cutting it turns into a delightful butterfly – a diamond worth hundreds, thousands and even millions of dollars.

But it’s not only the unearthly radiance and fantastic price that makes this stone stand out among its peers. Diamond is the hardest of all minerals, which determines the widest scope of its application. Not every diamond can turn into a diamond – only the purest and largest stones deserve this honor.

But even a small and cloudy nugget will not be thrown away as unnecessary, but will find application in the watch or nuclear industry, quantum computers or microelectronics, and, at worst, in the production of abrasive, drilling and cutting equipment. It’s Diamond!

General information about diamonds

Do you know the formula of diamond? Even a preschooler who has no idea about chemistry can remember it. It’s just C, which means diamonds are pure carbon (ideally, of course).

What had to happen for carbon to turn into diamond? Many hypotheses have been put forward on this matter. The most convincing of them claims that diamonds are formed at very great depths (over 200 km) and under tremendous pressure – there carbon forms a special cubic lattice inherent in diamonds. During volcanic processes, carbon crystals are carried closer to the surface, where they are discovered by diamond miners.

This process is very slow: the age of diamonds is measured in hundreds of millions, or even billions of years. So when, during intensive mining, diamond-bearing kimberlite pipes and other rocks are depleted, the reserves of this stone will dry up for a very long time.

According to scientific data, some diamonds are of extraterrestrial origin. They came to us with meteorites or came to us as a result of a supernova explosion. It is assumed that some of them are much older than the solar system!

There are a lot of diamonds on Earth, but only a tiny part of them can be turned into diamonds. The purest and largest diamonds (the so-called “Cape”) are mined in Africa, and Russian reserves of this mineral are concentrated mainly in Yakutia.

Among the most outstanding properties of diamond are the following:

  • unsurpassed hardness – 10 on the Mohs scale;
  • the highest thermal conductivity among solids – 900–2300 W/(m K);
  • vanishingly low coefficient of friction on metal (in air);
  • refractoriness and resistance to high temperatures;
  • resistance to most aggressive acids and alkalis;
  • high refractive index of rays combined with transparency;
  • the ability to luminesce (glow) in x-rays and ultraviolet light.

Diamonds are not only white, but also colored. Brown and yellow colors reduce the cost of a diamond, blue, blue, pink, red, green – increase it to sky-high heights.

The main characteristic that decides the fate of a rough diamond is transparency (“clear water”). That is why black diamonds (carbonados) have long been considered exclusively technical. However, occasionally you come across evenly colored black diamonds that retain some transparency and characteristic shine. They are mind-bogglingly expensive.

How is the hardness of a diamond measured?

Even a child knows that the strength of a diamond is incredible (meaning its hardness, not its resistance to shock). It is taken as the basic value on all measurement scales. And this is surprising, because diamond’s closest relatives, graphite and coal, which have the same elementary chemical composition, cannot boast of outstanding strength.

The secret of diamond hardness lies in the unique conditions of its formation: high temperatures and incredible pressure. With them, carbon atoms form a unique cubic crystal lattice. This determines the incredible hardness of the final substance, which can exist in natural conditions for billions of years!

Unsurpassed hardness allows diamond to be used in the production of equipment for drilling and ultra-precise cutting. No substance can resist the standard!

Mohs scale

The first successful attempt to create a scale of hardness of materials belongs to the German mineralogist Friedrich Mohs. Despite the fact that this system was presented to the scientific community back in 1811, it continues to be used to this day, mainly in application to minerals of natural origin (including precious stones).

The hardness of a diamond on the Mohs scale is 10, that is, this mineral was taken as an absolute: a priori there is nothing harder than it. The basis of this test is scratching. If a scratch remains on the surface of the test sample, then it is a priori softer than the standard.

The second place in hardness on the classical Mohs scale is held by corundum, which includes sapphires and rubies – 9 points. You can only scratch them with a diamond!

However, the very rare natural moissanite and its artificial analog carborundum (chemical formula SiC) have a strength of as much as 9,5 Mohs points. By the way, carborundum often replaces diamond both in industry and in the production of jewelry. Visually, it is practically indistinguishable from its noble brother, but costs an order of magnitude cheaper!

Everyone knows that diamond has greater strength than graphite, despite the identical chemical composition. However, not everyone knows that they are at diametrically opposite ends of the Mohs scale. The hardness of graphite is comparable to that of talc, and this is only one!

Rockwell system

With the advent of synthetic materials and super-hard alloys, the generally accepted Mohs scale has become inconvenient. Many systems were proposed, but in the metallurgical industry the Rockwell scale (more precisely, the Rockwells, because there were two of them, distant relatives with the same surname) took root most of all.

The Rockwell hardness of diamond is not measured – it is taken as a standard and the main working tool. The Rockwell measuring machine visually resembles a sewing machine, but instead of a needle, a diamond cone is used, and fabric replaces the material being tested.

The sample is exposed to a diamond cone with a given pressure for several seconds, then the parameters of the dent are assessed on a letter-numeric scale.

What’s harder than diamond?

Many attempts have been made to create or find in nature a material stronger than diamond. So far they have not been successful: obsidan, titanium, super-hard alloys, all kinds of innovative materials cannot compete with the noble standard. Moreover: many chemists and physicists even argue that substances stronger than diamond (more precisely, harder) cannot exist.

The most famous and scandalous story involves a substance called lonsdaleite, which in a chemical and physical sense is a hexagonal diamond. In the 60s of the last century, this mineral was synthesized artificially, and a little later it was discovered in small quantities in meteorite craters.

In 2009, a group of Chinese scientists published a sensational paper claiming that lonsdaleite is more than half as hard as cubic diamond (as we know it). Unfortunately, these data turned out to be a hoax and were not confirmed even by the calculations in the above work.

The most successful attempt to create a substance harder than diamond was made quite recently, in 2021. A duo of American scientists managed to produce hexagonal diamonds from graphite using targeted explosions. The resulting samples demonstrated better sound conductivity than classic cubic diamond, which theoretically indicates greater hardness.

Unfortunately, it has not yet been possible to test the theoretical calculations of American scientists experimentally. And the disgraced lonsdaleite, obtained from graphite by exposure to colossal pressure, shows a strength of only 7-8 points on the Mohs scale. And it’s unlikely to be used: it consists of crystals visible only under a microscope, and obtaining this substance is fantastically expensive.

There are other substances that are not much inferior to diamond in hardness: fullerites, all kinds of boron compounds, carbyne, and so on. They are slightly softer than diamond, but often surpass it in other characteristics: strength, resistance to chemical attack and ultra-high temperatures.

A stronger substance can be created from cubic diamond (for example, using nanoengineering). The Japanese succeeded, but how to process this unprecedentedly hard material?

Can a diamond be scratched or broken with a hammer?

Based on the above, we can conclude: it is impossible to scratch a diamond. This makes it possible to quickly identify rough glass counterfeits that are easily scratched by a steel needle or nail file. True, imitation diamonds like cubic zirconia, and even more so carborundum, cannot be recognized in this way.

But there are plenty of substances stronger than diamond – and even steel! This means breaking a diamond is quite possible. Naturally, the stone will calmly survive a fall from a considerable height, and even if you step on it, nothing critical will happen. But if you hit the diamond with force with a hammer, it will crack, otherwise it will crumble into small crumbs.

But you shouldn’t do this: the experiment will be too expensive, and the world will lose another diamond that has been formed over hundreds of millions of years!

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