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26-425: A facet is a flat surface of a geometric shape, e.g., of a cut gemstone. Facet may also refer to: Facet Of the hundreds of facet arrangements that have been used, the most famous is probably the round brilliant cut , used for diamond and many colored gemstones. This first early version of what would become the modern Brilliant Cut is said to have been devised by an Italian named Peruzzi, sometime in

52-491: A jamb peg machine used wooden dop sticks of precise length and a "mast" system consisting of a plate with holes carefully placed in it. By placing the back end of the dop into one of the many holes, the stone could be introduced to the lap at precise angles. These machines took considerable skill to operate effectively. Another method of facet cutting involves the use of cylinders to produce curved, concave facets. This technique can produce many unusual and artistic variations of

78-417: A crucial role in the outcome of a gem. While the general facet arrangement of a particular gemstone cut may appear the same in any given gem material, the angles of each facet must be carefully adjusted to maximize the optical performance. The angles used will vary based on the refractive index of the gem material. When light passes through a gemstone and strikes a polished facet, the minimum angle possible for

104-420: A longer and much weaker van der Waals bond . This gives graphite a single direction of cleavage, parallel to the basal pinacoid. So weak is this bond that it is broken with little force, giving graphite a slippery feel as layers shear apart. As a result, graphite makes an excellent dry lubricant . While all single crystals will show some tendency to split along atomic planes in their crystal structure , if

130-459: A low surface energy . The planes with the lowest energy will form the largest facets, in order to minimize the overall thermodynamic free energy of the crystal. If the surface energy as a function of the planes is known, the equilibrium shape of the crystal may be found via the Wulff construction . Growth conditions, including the surface the crystal is growing on top of (the substrate), may change

156-407: A polishing compound produce abrasions smaller than the wavelengths of light, thus making the minute scratches invisible. Since gemstones have two sides (the crown and pavilion), a device often called a "transfer jig" is used to flip the stone so that each side may be cut and polished. Cleaving relies on planar weaknesses of the chemical bonds in the crystal structure of a mineral. If a sharp blow

182-438: A set of possible "low-energy planes", which are usually planes on which the atoms are close-packed. For instance, a cubic crystal may have low-energy planes on the faces of the cube or on the diagonals. The planes are low-energy in the sense that if the crystal is cleaved along these planes, there will be relatively few broken bonds and a relatively small increase in energy over the unbroken crystal. Equivalently, these planes have

208-420: A wide variety of materials are used for polishing laps in conjunction with either very fine diamond powder or oxide-based polishes. Water is typically used for cutting, while either oil or water is used for the polishing process. The machine uses a system generally called a "mast" which consists of an angle readout, height adjustment and typically a gear (called an "index gear") with a particular number of teeth

234-456: Is applied at the correct angle, the stone may split cleanly apart. While cleaving is sometimes used to split uncut gemstones into smaller pieces, it is never used to produce facets. Cleaving of diamonds was once common, but as the risk of damaging a stone is too high, undesirable diamond pieces often resulted. The preferred method of splitting diamonds into smaller pieces is now sawing . An older and more primitive style of faceting machine called

260-410: Is bonded to four others in a tetrahedral pattern with short covalent bonds . The planes of weakness (cleavage planes) in a diamond are in four directions, following the faces of the octahedron . In graphite, carbon atoms are contained in layers in a hexagonal pattern where the covalent bonds are shorter (and thus even stronger) than those of diamond. However, each layer is connected to the other with

286-414: Is commonly referred to as "fire", and brightly colored flashes of reflected light known as scintillation . Typically transparent to translucent stones are faceted, although opaque materials may occasionally be faceted as the luster of the gem will produce appealing reflections. Pleonaste (black spinel ) and black diamond are examples of opaque faceted gemstones. The angles used for each facet play

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312-499: Is especially common in poorly cut commercial gemstones. Gemstones with higher refractive indexes generally make more desirable gemstones, the critical angle decreases as refractive indices increase, allowing for greater internal reflections as the light is less likely to escape. This machine uses a motor-driven plate to hold a precisely flat disk (known as a " lap ") for the purpose of cutting or polishing. Diamond abrasives bonded to metal or resin are typically used for cutting laps, and

338-412: Is still interest in them from collectors. Other historic diamond cuts include the "Old Mine Cut" which is similar to early versions of the round brilliant, but has a rectangular outline, and the " Rose Cut " which is a simple cut consisting of a flat, polished back, and varying numbers of angled facets on the crown, producing a faceted dome. Sometimes a 58th facet, called a culet is cut on the bottom of

364-411: Is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye. If bonds in certain directions are weaker than others, the crystal will tend to split along

390-399: Is used as a means of setting the rotational angle. The angles of rotation are evenly divided by the number of teeth present on the gear, though many machines include additional means of adjusting the rotational angle in finer increments, often called a "cheater". The stone is bonded to a (typically metal) rod known as a "dop" or "dop stick" and is held in place by part of the mast referred to as

416-423: The "quill". The dopped stone is ground at precise angles and indexes on cutting laps of progressively finer grit, and then the process is repeated a final time to polish each facet. Accurate repetition of angles in the cutting and polishing process is aided by the angle readout and index gear. The physical process of polishing is a subject of debate. One commonly accepted theory is that the fine abrasive particles of

442-421: The basal parting in pyroxenes . Cleavage is a physical property traditionally used in mineral identification, both in hand-sized specimen and microscopic examination of rock and mineral studies. As an example, the angles between the prismatic cleavage planes for the pyroxenes (88–92°) and the amphiboles (56–124°) are diagnostic. Crystal cleavage is of technical importance in the electronics industry and in

468-413: The cause is different. Cleavage occurs because of design weakness while parting results from growth defects (deviations from the basic crystallographic design). Thus, cleavage will occur in all samples of a particular mineral, while parting is only found in samples with structural defects. Examples of parting include the octahedral parting of magnetite , the rhombohedral and basal parting in corundum , and

494-413: The cutting of gemstones . Precious stones are generally cleaved by impact, as in diamond cutting . Synthetic single crystals of semiconductor materials are generally sold as thin wafers which are much easier to cleave. Simply pressing a silicon wafer against a soft surface and scratching its edge with a diamond scribe is usually enough to cause cleavage; however, when dicing a wafer to form chips,

520-486: The differences between one direction or another are not large enough, the mineral will not display cleavage. Corundum , for example, displays no cleavage. Cleavage forms parallel to crystallographic planes: Crystal parting occurs when minerals break along planes of structural weakness due to external stress, along twin composition planes, or along planes of weakness due to the exsolution of another mineral. Parting breaks are very similar in appearance to cleavage, but

546-484: The expected shape of the crystal; for instance, if the base of the crystal is under stress from the substrate, this may favor the crystal growing taller rather than growing outwards along the substrate. The surface energy, including the relative energies of the different planes, depend on many factors including the temperature, the composition of the surroundings (e.g. humidity), and the pressure. Cleavage (crystal) Cleavage , in mineralogy and materials science ,

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572-416: The facet to reflect the light back into the gemstone is called the critical angle . If the ray of light strikes a surface lower than this angle, it will leave the gem material instead of reflecting through the gem as brilliance. These lost light rays are sometimes referred to as "light leakage", and the effect caused by it is called "windowing" as the area will appear transparent and without brilliance. This

598-461: The late 17th century. Later on, the first angles for an "ideal" cut diamond were calculated by Marcel Tolkowsky in 1919. Slight modifications have been made since then, but angles for "ideal" cut diamonds are still similar to Tolkowsky's formula. Round brilliants cut before the advent of "ideal" angles are often referred to as "Early round brilliant cut" or "Old European brilliant cut" and are considered poorly cut by today's standards, though there

624-480: The stone to help prevent chipping of the pavilion point. Earlier brilliant cuts often have very large culets, while modern brilliant cut diamonds generally lack the culet facet, or it may be present in minute size. The art of cutting a gem is an exacting procedure performed on a faceting machine . The ideal product of facet cutting is a gemstone that displays a pleasing balance of internal reflections of light known as brilliance , strong and colorful dispersion which

650-402: The traditional faceting process. Many crystals naturally grow in faceted shapes. For instance, common table salt forms cubes and quartz forms hexagonal prisms. These characteristic shapes are a consequence of the crystal structure of the material and the surface energy , as well as the general conditions under which the crystal formed. The Bravais lattice of the crystal structure defines

676-417: The weakly bonded planes. These flat breaks are termed "cleavage". The classic example of cleavage is mica , which cleaves in a single direction along the basal pinacoid , making the layers seem like pages in a book. In fact, mineralogists often refer to "books of mica". Diamond and graphite provide examples of cleavage. Each is composed solely of a single element , carbon . In diamond, each carbon atom

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