Diamonds with rare and beautiful coloring are especially valued. Thanks to scientific and technological progress in the second half of the XX century it became possible to change the color of natural diamonds. Bombardment of diamond crystals by electrons, protons, neutrons and subsequent thermal treatment gives them yellow, blue, green, brown and smoky colors. Diamonds irradiated in a nuclear reactor acquire green and brown colors, and the elementary particles placed in the accelerator become blue or blue. Depending on the nature and intensity of the irradiation, the change in colour may occur only in the surface layer or in the entire volume of the crystal, it may disappear after a short time, it may disappear after a short time, or it may persist for years.
Crystals found in nature rarely take the form of regular polyhedrons. Usually their faces are not evenly developed, they have cracks, strokes, built-up areas, and foreign inclusions are not uncommon. That's why natural crystals usually do not have light in their play, and even before the invention of the method of cutting and polishing diamonds they did not have the price that they acquired later on. In the olden days, transparent octahedral diamond crystals with mirror-smooth faces were the most valued. According to legend, such diamonds were decorated with the gown of Louis the Saint.
Even in ancient India it was noticed that the friction of one diamond on the other side of them are polished and the shine increases. Some time later, in India, and later in Italy, France and Belgium, diamond cutting was applied as a "platform" or "octahedron". For such a simple cut, natural octahedral crystals were taken or blocks of the appropriate shape were poked out of diamond crystals of other shapes.
The real beauty, shine and enchanting "game" of rays of light and diamonds are revealed and achieved by special mechanical processing of natural transparent crystals, which are then called diamonds. Large diamonds are called solitaires. Processing consists in splitting or sawing, followed by pointing and cutting the crystals from all sides to give them a special shape.
Diamond splitting makes it possible to divide the crystals into parts for more efficient use with little loss of raw materials and little labor, in particular, to get rid of defective and foreign parts of the crystal. This operation requires a great deal of skill, because even with a single negligent blow, a diamond can be turned into pieces that are unsuitable for making diamonds.
Until the beginning of the 20th century, diamonds were ground by hand. For many weeks and months, a person has been grinding a workpiece with great effort for the future of the diamond. However, this way, even with the high skill and diligence of the employee, it was extremely difficult to ensure the correct geometric shape of the workpiece.
At the beginning of the 20th century, the diamond point machine was invented, resulting in a dramatic improvement in the quality of processing and productivity. The working bodies of the first machines were driven into rotation by foot pedals and later by electric motors. In the second half of the XX century, the appearance of the machines also underwent significant changes.
Cutting is the final process of diamond processing in order to give them an aesthetic shape, to achieve the characteristic shine and "light a game", as well as to eliminate cracks, stakes and other surface or near surface defects. Grinding consists in giving the surface of the workpiece lawfully arranged faces of a certain shape, polishing provides a mirror-smooth surface on the facets obtained during grinding. Cutting is rightly considered to be the most complicated and responsible process in diamond manufacturing. In addition to knowledge and experience, its successful implementation also requires an artistic taste. Cutting is done with the help of a fast rotating cast iron disc, in which the diamond powder is rubbed into the surface, diluted in turnip or olive oil. Thus the form of the received polyhedron as a whole and a mutual arrangement of sides is made with such expectation that the most part of submitted light and penetrated inside, but did not pass through, and would return back.
The diamond not only refracts and reflects the light rays very strongly, but also has another very important optical property that determines the exceptional beauty of this stone. So, if for red light and the refractive index is 2.402, for violet rays it reaches 2.465. The difference between the light refraction indexes of violet and red rays (dispersion) is 5 times greater in diamond than in rock crystal, and 2 times greater than the corresponding characteristic of the best honeycomb glasses. Due to the high dispersion, diamonds have a strong property of decomposition of white into the colors of the rainbow. For this reason, the same stone seems to be colored in different colors depending on the position of the light source and the observer.