Diamond is a natural mineral, one of the most famous and expensive. Around it goes a lot of speculation and legends, especially with regard to its value and the identification of fakes. A separate topic for study is the relationship of diamond and graphite. Many people know that these minerals are similar, but not everyone knows what exactly. And not everyone will be able to answer the question of how they differ. And what do we know about the structure of diamond? Or about the criteria for evaluating gems?
Diamond structure
Diamond is one of three minerals that are crystalline modifications 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 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 that the atoms are arranged in the form of tetrahedrons, and each atom is in the center of one of them. Particles are interconnected by the strongest bond - covalent, due to which diamond has high hardness.
Chemical properties
Roughly speaking, a diamond is pure carbon; accordingly, diamond crystals must be absolutely transparent and transmit all visible light. But in the world there is nothing ideal, which means that this mineral also has impurities. It is believed that the maximum content of impurities in jewelry diamonds should not exceed 5%. The composition of diamond may 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 contained in 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 cell.
The hardness of minerals is evaluated on the Mohs scale, its rating on this system is 10, this is the maximum indicator. The next corundum in the list, its indicator is 9, but its hardness is 150 times less, which means the absolute superiority of diamond in this indicator.
However, the hardness of a mineral does not at all mean its strength. The diamond is quite fragile and easily breaks if you hit it with a hammer.
The specific gravity of diamond (density) is determined in the range from 3.42 to 3.55 g / cm 3 . It is determined by the ratio of the weight of the mineral to the weight of water of the same volume.
In addition to hardness, it also has high refractive indices of light (2.417-2.421) and dispersion (0.0574). This combination of properties allows the diamond to be the most precious and perfect jewelry stone.
Other physical properties of the mineral, such as thermal conductivity (900-2300 W / m · K), also the highest of all substances, are also of importance. We can also note the ability of the mineral to not dissolve in acids and alkalis, dielectric properties, low coefficient of friction on a metal in air and a high melting point of 3700-4000 ° C at a pressure of 11 GPa.
Similarities and differences of 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, consists 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 of oxygen, but under normal conditions it is able to remain unchanged indefinitely, this is called metastability, and the type of diamond crystal lattice is 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 forming a honeycomb-like system. Strong bonds are formed only between these hexagons, but between the layers they are extremely weak, this determines the layering of the mineral. In addition to 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 one chemical element.
Origin of diamond
There is no unequivocal opinion on how diamonds are formed in nature; there are magmatic, mantle, meteorite and other theories. However, the most common is igneous. It is believed that diamonds are formed at a depth of about 200 km under a pressure of 50,000 atmospheres, and after that they are carried to the surface together 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 form when a meteorite hits the surface of the earth, as well as being in the meteorite rock itself. However, crystals of this origin are extremely small and rarely suitable for processing.
Diamond deposits
The first deposits in which diamonds were discovered and mined were 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 XVII and XIX centuries, mineral deposits were discovered in Brazil and South Africa. History is replete with legends and facts about 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 XX century.
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, but indicates the high quality of minerals. Diamonds that have traveled several hundred kilometers to the surface with constant friction against other rocks are high-grade, less high-quality crystals simply could not withstand such a trip, and therefore 95% of the mined stones are of jewelry quality. Also known and mineral-rich kimberlite pipes are in Russia, Botswana, Angola, Guinea, Liberia, Tanzania and other countries.
Diamond processing
Diamond processing requires vast experience, knowledge and skills. Before starting work, it is necessary to thoroughly examine the stone in order to subsequently maximize its weight and get rid of inclusions. The most common type of diamond cutting is round, it allows the stone to sparkle with all the colors and reflect the light as profitably 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 accurate proportions. Also popular types of cut are: oval, tear, heart, marquise, emerald and others. There are several stages of processing minerals:
- markup;
- cracking;
- sawing;
- rounding;
- faceting.
It is still believed that after processing the diamond loses about half its weight.
Diamond Assessment Criteria
When mining diamonds, 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 double). Valuation of diamonds is carried out according to the 4C system:
- Carat (carat) - 1 carat is 0.2 g.
- Color - almost no pure white diamonds are found; most minerals have a certain shade. The value of the diamond depends on the color of the diamond, most of the stones found in nature have a yellow or brown hue, pink, blue and green stones are less often found. 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 (purity) is also an important indicator that determines the presence of defects in the stone and significantly affects its value.
- Cut - the appearance of a diamond is highly dependent on the cut. Refraction and reflection of light, a kind of "diamond" shine make this stone so valuable, and an irregular shape or aspect ratio during processing can completely ruin it.
Artificial Diamond Making
Now technology allows you to "grow" diamonds, almost indistinguishable from natural ones. There are several methods of synthesis:
- The creation of HPHT diamonds is the closest technique to environmental 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.
- The creation of CVD diamonds (film synthesis) - the manufacture of stones in vacuum conditions using seeds and methane and hydrogen gases. This method allows you to synthesize the purest minerals, however, extremely small in size, because basically they have an industrial purpose.
- Explosive synthesis - this method allows to obtain small crystals of diamond through the detonation of explosives and subsequent cooling.
How to distinguish an original from a fake
Speaking of methods for determining the authenticity of diamonds, it is worth distinguishing between authenticating diamonds and rough diamonds. An inexperienced person can confuse a diamond with quartz, crystal, other transparent minerals, and even glass. Nevertheless, the exceptional physical and chemical properties of diamond make it easy to identify fakes.
First of all, it is worth recalling hardness. This stone can scratch any surface, but only another diamond can leave marks on it. Also, there is no evaporation on a natural crystal, if you breathe on it. On a wet stone there will be a trace like a pencil, if you draw on it with aluminum. You can check it by X-ray: the natural stone under the radiation has a saturated green color. Or look through it at the text: through a natural diamond it will be impossible to make out. Separately, it is worth noting that the naturalness of the stone can be checked for refraction of light: by raising the original to the light source, you can only see the luminous point in the center.