Diamond grinding - all about the perfect cut of diamonds

The origins of the great art of polishing precious stones go back to ancient times. Already the Sumerians, Assyrians and Akkids boasted of beautiful ornaments and amulets, in which precious stones were set, still round and not very outlined, but beautifully polished. Material for whetstones was given to man by nature itself, showing the shiny surfaces of many correctly formed crystals. Man, imitating nature, the grinding process, through the use of technology, only accelerated and improved, awakening the potential beauty of stones as if from a dream.

The first attempts to polish diamonds date back to the XNUMXth century, and the form of the brilliant cut, still imperfect, to the XNUMXth century It is thanks to these cuts, thanks to strictly defined proportions, that we can now admire the many wonderful optical effects of diamonds, which gemologists call brilliance.

Forms of education

Mineralogically, diamond is pure carbon (C). It crystallizes in the correct system, most often in the form of octahedrons (Fig. 1), less often tetra-, six-, twelve-, and very rarely octahedrons (Fig. 1). Of course, under natural conditions, perfectly formed pure crystals are rare and usually very small. Larger crystals are most often morphologically poorly developed (photo 2). Many of them have a mosaic structure as a result of multiple twins or adhesions; many crystals have rounded edges, and the walls are convex, rough, or jagged. There are also deformed or etched crystals; their formation is closely related to the conditions of formation and subsequent dissolution (surface etching). Spinel-type twins are common forms, in which the plane of fusion is the plane of the octahedron (111). Multiple twins are also known, forming star-shaped figures. There are also irregular adhesions. Examples of the most common forms in nature are shown in fig. 2. There are gem diamonds (the purest, almost perfect crystals) and industrial diamonds, which are subdivided into boards, carbonados, ballas, etc. according to mineralogical characteristics. Board (board, board) is usually in the form of granular clusters, gray or black. Ballas are accumulations of grains, most often of a radiant structure and gray color. Carbonado, also known as black diamond, is cryptocrystalline."Total diamond production since ancient times is estimated at 4,5 billion carats, with a total value of $300 billion."

Diamond grinding

The origins of the great art of polishing diamonds date back to ancient times. It is known that the Sumerians, Assyrians and Babylonians already boasted cut stones used as jewelry, amulets or talismans. It is also known that the grinding stones were stimulated by nature itself, showing the surfaces of many well-formed crystals shining with brilliance, or water-smoothed pebbles with a strong luster and characteristic color. Thus, they imitated nature by rubbing less hard stones with harder ones, giving them a round, but asymmetrical, irregular shape. The polishing of stones to a symmetrical shape came much later. Over time, the modern cabochon shape evolved from rounded shapes; There are also flat surfaces on which engraving is made. Interestingly, the processing of stones with symmetrically arranged faces (facets) was known much later than the engraving of stones. The flat stones with symmetrically arranged walls, which we admire today, originate only in the Middle Ages. 

Stages of polishing diamonds

In the process of processing diamonds, cutters stand out 7 stages.First stage - the preparatory stage, at which the rough diamond is subjected to a detailed examination. The most important factors are the shape and type of the crystal, its purity and color. The simple shapes of diamonds (cube, octahedron, rhombic dodecahedron) are clearly distorted in natural conditions. Rarely, diamond crystals are limited to flat faces and straight edges. They are usually rounded to varying degrees and create uneven surfaces. Convex, concave or skeletal forms predominate. At the same time, in addition to simple, more or less distorted forms, complex forms can also arise, which are a combination of simple forms or their twins. It is also possible the appearance of distortedly deformed crystals, which have largely lost their original shape of a cube, octahedron or rhombic dodecahedron. Therefore, it is necessary to thoroughly know all these deformation defects that can affect the subsequent course of the processing process, and plan the process in such a way that the yield of cut diamonds is as high as possible. The color of diamonds is indirectly related to the shape of the crystals. Namely, it was found that orthorhombic dodecahedrons are mostly yellow in color, while octahedrons are usually colorless. At the same time, in many crystals, color inhomogeneity can occur, consisting in zonal and clearly different color saturation. Therefore, the precise determination of these differences also has a significant impact on the processing and subsequent quality of polished stones. The third important factor to be determined at the preliminary stage is the purity of the rough diamond. Therefore, the type and nature of inclusions, size, form of formation, quantity and distribution in the crystal are investigated. It also determines the location and extent of chip marks, fracture cracks and stress cracks, i.e. all structural disturbances that can affect the grinding process and affect the subsequent assessment of the quality of the stone. Currently, computed tomography methods have proven to be extremely useful in this regard. These methods, thanks to the use of an appropriate device, give a three-dimensional image of a diamond with all its internal defects, thanks to which, by means of computer simulation, all operations associated with the grinding process can be accurately programmed. A significant obstacle to the spread of this method is, unfortunately, the high cost of the device, which is why many grinders still use traditional methods of visual inspection, using a small flat “window” for this purpose, previously polished on one of the facets of the crystal.The second stage - cracking of crystal. This operation is usually carried out on underdeveloped, deformed, twinned or heavily contaminated crystals. This is an activity that requires a lot of knowledge and experience. The bottom line is to divide the crystal in such a way that its parts are not only as large as possible, but also as clean as possible, that is, the suitability for further processing should be correlated with the stones being processed. Therefore, when splitting, more and more attention is paid not only to potential separation surfaces (cleavage planes), but also to the simultaneous possibility of eliminating various kinds of external and internal defects, such as cracks, twin planes, clear traces of cleavage, significant inclusions, etc. It is worth recalling that that diamond is characterized by octahedral cleavage (along the (111) plane), and therefore the potential partition surfaces are the planes of the octahedron. Of course, the more accurate their definition is, the more efficient and reliable the whole operation will be, especially considering the high fragility of diamond.The third stage – sawing (crystal cutting). This operation is performed on large well-formed crystals in the form of a cube, octahedron and orthorhombic dodecahedron, provided that the division of the crystal into parts has been planned in advance. For cutting, special saws (saws) with phosphor bronze discs are used (photo 3).Step four - initial grinding, which consists in the formation of a figure (Fig. 3). A rondist is formed, that is, a strip separating the upper part (crown) of the stone from its lower part (pavilion). In the case of a brilliant cut, the rondist has a round outline.Stage Five - correct grinding, which consists in grinding the front side of the stone, then the collet and the main faces of the crown and pavilion (photo 4). The process completes the formation of the remaining faces. Before the start of cutting operations, stones are selected to determine the directions of cutting, which is associated with the existing anisotropy of hardness. The general rule when polishing diamonds is to keep the surface of the stone parallel to the walls of the cube (100), the walls of the octahedron (111) or the walls of the diamond dodecahedron (110) (Fig. 4). Based on this, three types of rhombuses are distinguished: a four-pointed rhombus (Fig. 4a), a three-pointed rhombus (Fig. 4b) and a two-pointed rhombus (Fig. 5), fig. in). It has been experimentally established that it is easiest to grind the planes parallel to the fourfold symmetry axis. Such planes are the faces of the cube and the rhombic dodecahedron. In turn, the planes of the octahedron inclined to these axes are the most difficult to grind. And since most of the grinded faces are only extremely parallel to the fourth-order symmetry axis, the grinding directions are chosen that are closest to one of these axes. The practical use of the anisotropy of hardness on the example of a brilliant cut is shown in fig. XNUMX.Sixth stage - polishing, which is a continuation of grinding. Suitable polishing discs and pastes are used for this.Seventh stage - checking the correctness of the cut, its proportions and symmetry, and then cleaning by boiling in a solution of acids, mainly sulfuric acids.

Weight increase

The mass yield of crushed diamond crystals depends on their shape (shape), and the mass spread can be significant. This is confirmed by the calculated data, according to which the yield of diamonds cut from correctly formed shapes is about 50–60% of the initial mass, while with clearly deformed shapes it is only about 30%, and with flat shapes, a twins are only about 10– 20% (photo 5, 1-12).

STRAIGHT ANT BRILLIARIA

rosette cut

The rosette cut is the first cut to use flat facets. The name of this form comes from the rose; is the result of associating a certain similarity in the arrangement of facets in the stone with the arrangement of the petals of a well-developed rose. The rosette cut was widely used in the 6th century; at present, it is rarely used and mainly when processing small fragments of stones, the so-called. makle. In the Victorian era, it was used to grind deep red garnet, which was very fashionable at the time. Faceted stones have only a faceted upper part, while the lower part is a flat polished base. The upper part is shaped like a pyramid with triangular faces converging at a greater or lesser angle towards the top. The simplest forms of rosette cutting are shown in fig. 7. Other types of rosette cutting are currently known. These include: the full Dutch rosette (fig. 7 a), the Antwerp or Brabant rosette (fig. XNUMX b) and many others. In the case of a double form, which can be described as a basic connection of two single forms, a double Dutch socket is obtained.

Cutting tiles

This is probably the first faceted cut adapted to the octagonal shape of the diamond crystal. Its simplest form resembles an octahedron with two truncated vertices. In the upper part, the glass surface is equal to half the cross section of the octahedron in its widest part, in the lower part it is half as much. Tile cutting was widely used by the ancient Indians. It was brought to Europe in the second half of the 8th century by Nuremberg grinders. There are many types of board cut, among which are the so-called Mazarin cut (Fig. 8a) and Peruzzi (Fig. XNUMXb), widespread in France and Italy in the XNUMXth century. At present, tile cutting is mainly used in very fine form; Stones cut in this way act as coverslips for various miniatures embedded, for example, in rings.

stepped cut

The prototype of this form of cutting, now very common, was the tile cut. It is characterized by a large flat surface (panel) surrounded by a series of rectangular facets resembling steps. In the upper part of the stone, the facets grow gradually, descending steeply to its widest edge; in the lower part of the stone, the same rectangular facets are visible, stepwise descending to the lower face of the base. The outline of the stone can be square, rectangular, triangular, rhombic or fancy: kite, star, key, etc. A rectangular or square cut with cut corners (an octagonal contour of the stone in the rondist plane) is called an emerald cut (Fig. 9). Small stones, stepped and elongated, rectangular or trapezoidal, are known as baguettes (French baquette) (Fig. 10 a, b); Their variety is a square step-cut stone called carré (Fig. 10c).

Old brilliant cuts

In jewelry practice, it often happens that diamonds have a cut that is significantly different from the “ideal” proportions. Most often, these are old-cut diamonds made in the 11th century or earlier. Such diamonds do not show such remarkable optical effects as those that are cut today. Diamonds of the old brilliant cut can be divided into two groups, the turning point here being the middle of the nineteenth century. Diamonds of the earlier period usually have a stone shape similar to a square (called cushion), with more or less convex sides. , a characteristic arrangement of faces, a very large base and a small window (Fig. 12). Diamonds cut after this period also have a small surface and a large truncated collet, however, the outline of the stone is round or close to round and the arrangement of facets is quite symmetrical (Fig. XNUMX).

BRILLIANT CUT

The vast majority of brilliant cut is used for diamonds, so the name "brilliant" is often considered synonymous with the name of the diamond. The brilliant cut was invented in the 13th century (some sources suggest that it was known as early as the 33rd century) by the Venetian grinder Vincenzio Peruzzi. The modern term "diamond" (Fig. 25, a) denotes a round shape with 1 facets in the upper part (crown), including glass, and in the lower part (pavilion) with 8 faces, including collets. The following faces are distinguished: 8) in the upper part (crown) - a window, 16 faces of the window, 13 main faces of the crown, 2 faces of the rondist crown (Fig. 8 b); 16) in the lower part (pavilion) - 13 main faces of the pavilion, XNUMX faces of the rondist pavilion, tsar (Fig. XNUMX c). The strip separating the upper and lower parts is called rondist; it provides protection against damage to the converging edges of the facets. 

Check also our compendium of knowledge about other gems:

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