Diamond is a natural mineral, one of the most famous and expensive. There are many speculations and legends around him, especially with regard to its value and the detection of fakes. A separate topic for study is the relationship between diamond and graphite. Many people know that these minerals are similar, but not everyone knows exactly what. And the question of how they differ, too, not everyone can answer. What do we know about the structure of a diamond? Or the criteria for judging gems?
Diamond structure
Diamond is one of three minerals that are a crystalline modification of carbon. The other two are graphite and lonsdaleite, the second can be found in meteorites or created artificially. And if these stones are hexagonal modifications, then the type of the diamond crystal lattice is a cube. In this system, carbon atoms are arranged in this way: one at each vertex and in the center of the face, and four inside the cube. Thus, it turns out thatthe atoms are arranged in the form of tetrahedra, and each atom is in the center of one of them. The particles are interconnected by the strongest bond - covalent, due to which the diamond has a high hardness.
Chemical properties
Roughly speaking, a diamond is pure carbon, therefore, diamond crystals must be absolutely transparent and transmit all visible light. But there is nothing perfect in the world, which means that this mineral also has impurities. It is believed that the maximum content of impurities in gem diamonds should not exceed 5%. The composition of a diamond can include both solid and liquid and gaseous substances, the most common of them:
- nitrogen;
- boron;
- aluminum;
- silicon;
- calcium;
- magnesium.
Also, the composition may include quartz, garnets, olivine, other minerals, iron oxides, water and other substances. Often these elements are found in the composition of the mineral in the form of mechanical mineral inclusions, but some of them can replace carbon in the diamond structure - this phenomenon is called isomorphism. In this case, inclusions can significantly affect the physical properties of the mineral, its color, light reflection, and nitrogen inclusions give it luminescent properties.
Physical properties
The structure of a diamond determines its physical properties, they are evaluated according to four criteria:
- hardness;
- density;
- dispersion and refraction of light;
- crystal lattice.
The hardness of minerals is measured on the Mohs scale, its score on this system is 10, this is the maximum indicator. Corundum is next on the list, its index is 9, but its hardness is 150 times less, which means that diamond is absolutely superior in this indicator.
However, the hardness of a mineral does not mean its strength at all. Diamond is quite brittle and breaks easily if hit with a hammer.
The specific gravity of a diamond (density) is determined in the range from 3.42 to 3.55 g/cm3. It is determined by the ratio of the weight of the mineral to the weight of water of the same volume.
Besides hardness, it also has high refractive indices (2.417-2.421) and dispersion (0.0574). This combination of properties allows the diamond to be the most precious and ideal jewelry stone.
The other physical properties of the mineral are also important, such as thermal conductivity (900-2300 W/m·K), also the highest of all substances. You can also note the ability of the mineral not to dissolve in acids and alkalis, the properties of the dielectric, the low coefficient of friction for metal in air and the high melting point of 3700-4000 °C at a pressure of 11 GPa.
Similarities and differences between diamond and graphite
Carbon is one of the most common elements on Earth, it is found in many substances, especially in living organisms. Graphite, like diamond, is made up of carbon, but the structures of diamond and graphite are very different. Diamond can turn into graphite under the influence of high temperatures without access to oxygen, but under normal conditions, it is capable of indefinitelyremain unchanged, this is called metastability, in addition, the type of diamond crystal lattice is a cube. But graphite is a layered mineral, its structure looks like a series of layers located in different planes. These layers are made up of hexagons that form a honeycomb-like system. Strong bonds are formed only between these hexagons, but between the layers they are extremely weak, this causes the layering of the mineral. In addition to its low hardness, graphite absorbs light and has a metallic luster, which is also very different from diamond.
These minerals are the most striking example of allotropy - a phenomenon in which substances have different physical properties, although they consist of the same chemical element.
Origin of diamond
There is no unequivocal opinion about how diamonds are formed in nature, there are magmatic, mantle, meteorite and other theories. However, the most common is magmatic. It is believed that diamonds are formed at a depth of about 200 km under a pressure of 50,000 atmospheres, and then brought to the surface along with magma during the formation of kimberlite pipes. The age of diamonds varies from 100 million to 2.5 billion years. It is also scientifically proven that diamonds can be formed when a meteorite hits the earth's surface, and also be found in the meteorite rock itself. However, crystals of this origin are extremely small and rarely suitable for processing.
Diamond deposits
The first deposits that werediamonds were discovered and mined, located in India, but by the end of the 19th century they were severely depleted. However, it was there that the most famous, large and expensive samples were mined. And in the 17th and 19th centuries, mineral deposits were discovered in Brazil and South Africa. History is replete with legends and facts about the diamond rush, which are associated specifically with South African mines. The last discovered diamond deposits are in Canada, their development began only in the last decade of the 20th century.
The mines of Namibia are especially interesting, although diamond mining there is a difficult and dangerous business. Deposits of crystals are concentrated under a layer of soil, which, although it complicates the work, speaks of the high quality of minerals. Diamonds that have traveled several hundred kilometers to the surface with constant friction against other rocks are high-grade, lower-quality crystals simply could not withstand such a journey, and therefore 95% of the stones mined are of gem quality. There are also well-known and mineral-rich kimberlite pipes in Russia, Botswana, Angola, Guinea, Liberia, Tanzania and other countries.
Diamond processing
Diamond processing requires great experience, knowledge and skills. Before starting work, it is necessary to thoroughly study the stone in order to subsequently preserve its weight as much as possible and get rid of inclusions. The most common type of diamond cut is round, it allows the stone to sparkle with all colors and reflect light as favorably as possible. But this work is also the most difficult:a round diamond has 57 planes, and when cutting it, it is important to observe the most precise proportions. Also popular types of cut are: oval, teardrop, heart, marquise, emerald and others. There are several stages of mineral processing:
- markup;
- splitting;
- sawing;
- rounding;
- cut.
It is still believed that after processing, the diamond loses about half of its weight.
Diamond evaluation criteria
When diamonds are mined, only 60% of the minerals are suitable for processing, they are called jewelry. Naturally, the cost of rough stones is much lower than the price of diamonds (more than twice). Diamonds are valued according to the 4C system:
- Carat (carat weight) - 1 carat equals 0.2 g.
- Color (color) - pure white diamonds are almost never found, most minerals have a certain shade. Its value largely depends on the color of the diamond, most of the stones found in nature have a yellow or brown tint, pink, blue and green stones can be found less often. The most rare, beautiful, and therefore expensive are minerals of saturated colors, they are called fantasy. The rarest ones are green, purple and black.
- Clarity (clarity) is also an important indicator that determines the presence of defects in the stone and significantly affects its value.
- Cut (cut) - the appearance of a diamond depends greatly on the cut. Refraction and reflection of lighta kind of "brilliant" radiance make this stone so valuable, and the wrong shape or ratio of proportions during processing can completely ruin it.
Manufacture of artificial diamonds
Now technology allows you to "grow" diamonds, almost indistinguishable from natural ones. There are several ways to synthesize:
- Creation of HPHT-diamonds is the method closest to natural conditions. Minerals are created from graphite and seed diamond at a temperature of 1400 ° C under a pressure of 50,000 atmospheres. This method allows you to synthesize gem-quality stones.
- Creation of CVD-diamonds (film synthesis) - the manufacture of stones in vacuum conditions using a seed and methane and hydrogen gases. This method makes it possible to synthesize the purest minerals, however, they are extremely small in size, therefore they are mainly used for industrial purposes.
- Explosive fusion is a method that produces small diamond crystals by detonating explosives and then cooling them down.
How to distinguish an original from a fake
Speaking of methods for determining the authenticity of diamonds, it is worth distinguishing between the verification of the authenticity of diamonds and rough diamonds. An inexperienced person can confuse a diamond with quartz, crystal, other transparent minerals, and even glass. However, the exceptional physical and chemical properties of a diamond make it easy to spot a fake.
BFirst of all, it is worth remembering the hardness. This stone is able to scratch any surface, but only another diamond can leave marks on it. Also, perspiration does not remain on a natural crystal if you breathe on it. On a wet stone, there will be a mark like a pencil if you run aluminum over it. You can check it with an x-ray: a natural stone under radiation has a rich green color. Or look through it at the text: it will be impossible to make out it through a natural diamond. Separately, it is worth noting that the naturalness of the stone can be checked for the refraction of light: bringing the original to the light source, you can see only a luminous dot in the center.
