A diamond cut is a style or design guide used when shaping a diamond for polishing such as the brilliant cut. Cut does not refer to shape, but the symmetry and polish of a diamond; the cut of a diamond affects a diamond's brilliance. In order to best use a diamond gemstone's material properties, a number of different diamond cuts have been developed. A diamond cut constitutes a more or less symmetrical arrangement of facets, which together modify the shape and appearance of a diamond crystal. Diamond cutters must consider several factors, such as the shape and size of the crystal, when choosing a cut; the practical history of diamond cuts can be traced back to the Middle Ages, while their theoretical basis was not developed until the turn of the 20th century. Design creation and innovation continue to the present day: new technology—notably laser cutting and computer-aided design—has enabled the development of cuts whose complexity, optical performance, waste reduction were hitherto unthinkable.
The most popular of diamond cuts is the modern round brilliant, whose facet arrangements and proportions have been perfected by both mathematical and empirical analysis. Popular are the fancy cuts, which come in a variety of shapes, many of which were derived from the round brilliant. A diamond's cut is evaluated by trained graders, with higher grades given to stones whose symmetry and proportions most match the particular "ideal" used as a benchmark; the strictest standards are applied to the round brilliant. Different countries base their cut grading on different ideals: one may speak of the American Standard or the Scandinavian Standard, to give but two examples; the history of diamond cuts can be traced to the late Middle Ages, before which time diamonds were employed in their natural octahedral state—anhedral diamonds were not used in jewelry. The first "improvements" on nature's design involved a simple polishing of the octahedral crystal faces to create and unblemished facets, or to fashion the desired octahedral shape out of an otherwise unappealing piece of rough.
This was called the point cut and dates from the mid 14th century. By the mid 15th century, the point cut began to be improved upon: a little less than one half of the octahedron would be sawn off, creating the table cut; the importance of a culet was realised, some table-cut stones may possess one. The addition of four corner facets created the old single cut. Neither of these early cuts would reveal. At the time, diamond was valued chiefly for its adamantine superlative hardness. For this reason, colored gemstones such as ruby and sapphire were far more popular in jewelry of the era. In or around 1476, Lodewyk van Berquem, a Flemish polisher of Bruges, introduced the technique of absolute symmetry in the disposition of facets using a device of his own invention, the scaif, he cut stones in the shape known as briolette. About the middle of the 16th century, the rose or rosette was introduced in Antwerp: it consisted of triangular facets arranged in a symmetrical radiating pattern, but with the bottom of the stone left flat—essentially a crown without a pavilion.
Many large, famous Indian diamonds of old feature a rose-like cut. However, Indian "rose cuts" were far less symmetrical as their cutters had the primary interest of conserving carat weight, due to the divine status of diamond in India. In either event, the rose cut continued to evolve, with its depth and arrangements of facets being tweaked; the first brilliant cuts were introduced in the middle of the 17th century. Known as Mazarins, they had 17 facets on the crown, they are called double-cut brilliants as they are seen as a step up from old single cuts. Vincent Peruzzi, a Venetian polisher increased the number of crown facets from 17 to 33, thereby increasing the fire and brilliance of the cut gem, properties that in the Mazarin were incomparably better than in the rose, yet Peruzzi-cut diamonds, when seen nowadays, seem exceedingly dull compared to modern-cut brilliants. Because the practice of bruting had not yet been developed, these early brilliants were all rounded squares or rectangles in cross-section.
Given the general name of cushion—what are known today as old mine cuts—these were common by the early 18th century. Sometime the old European cut was developed, which had a shallower pavilion, more rounded shape, different arrangement of facets; the old European cut was the forerunner of modern brilliants and was the most advanced in use during the 19th century. Around 1900, the development of diamond saws and good jewelry lathes enabled the development of modern diamond cutting and diamond cuts, chief among them the round brilliant cut. In 1919, Marcel Tolkowsky analyzed this cut: his calculations took both brilliance and fire into consideration, creating a delicate balance between the two. Tolkowsky's calculations would serve as the basis for all future brilliant cut modifications and standards. Tolkowsky's model of the "ideal" cut is not perfect; the original mo
A diamond is one of the best-known and most sought-after gemstones. Diamonds have been used as decorative items since ancient times. C; the hardness of diamond and its high dispersion of light—giving the diamond its characteristic "fire"—make it useful for industrial applications and desirable as jewelry. Diamonds are such a traded commodity that multiple organizations have been created for grading and certifying them based on the "four Cs", which are color, cut and carat. Other characteristics, such as presence or lack of fluorescence affect the desirability and thus the value of a diamond used for jewelry. Diamonds are used in engagement rings; the practice is documented among European aristocracy as early as the 15th century, though ruby and sapphire were more desirable gemstones. The modern popularity of diamonds was created by De Beers Consolidated Mines Ltd. which established the first large-scale diamonds mines in South Africa. Through an advertising campaign beginning in the 1930s and continuing into the mid-20th century, De Beers made diamonds into a key part of the betrothal process and a coveted symbol of status.
The diamond's high value has been the driving force behind dictators and revolutionary entities in Africa, using slave and child labor to mine blood diamonds to fund conflicts. Though popularly believed to derive its value from its rarity, gem-quality diamonds are quite common compared to rare gemstones such as alexandrite, annual global rough diamond production is estimated to be about 130 million carats. Before diamonds were discovered in Brazil in the 1700s, India was the only place where diamonds were mined. Early references to diamonds in India come from Sanskrit texts; the Arthashastra of Kautilya mentions diamond trade in India. Buddhist works dating from the 4th century BC describe the diamond as a well-known and precious stone but do not mention the details of diamond cutting. Another Indian description written in the beginning of the 3rd century describes strength, brilliance, ability to scratch metals, good refractive properties as the desirable qualities of a diamond. Kalkutta was an important trading center for diamonds in central India.
Diamonds were traded to the east and west of India and were recognized by various cultures for their gemmological or industrial uses. In his work Naturalis Historia, the Roman writer Pliny the Elder noted diamond's ornamental uses, as well as its usefulness to engravers because of its hardness, it is however doubtful that Pliny meant diamonds, it is assumed that in fact several minerals such as corundum, spinel, or a mixture with magnetite were all referred to by the word "adamas". Diamonds spread throughout the world though India had remained the only major source of the gemstone until diamonds were discovered in Brazil in 1725. A Chinese work from the 3rd century BC mentions: "Foreigners wear it in the belief that it can ward off evil influences"; the Chinese, who did not find diamonds in their country did not use diamond as a jewel but used it as a "jade cutting knife". Diamonds reached ancient Rome from India. Diamonds were discovered in 700 in Borneo, were used by the traders of southeast Asia.
The modern era of diamond mining began in the 1860s in Kimberley, South Africa with the opening of the first large-scale diamond mine. The first diamond there was found in 1866 on the banks of the Orange River and became known as the Eureka Diamond. In 1869, an larger 83.50-carat diamond was found on the slopes of Colesberg Kopje on the farm Vooruitzigt belonging to the De Beers brothers. This sparked off the famous "New Rush" and within a month, 800 claims were cut into the hillock which were worked frenetically by two to three thousand men; as the land was lowered so the hillock became a mine—in time, the world-renowned Kimberley Mine. Following agreement by the British government on compensation to the Orange Free State for its competing land claims, Griqualand West was annexed to the Cape Colony in 1877. From 1871 to 1914, 50,000 miners dug the Big Hole with picks and shovels, yielding 2,722 kg of diamonds, by 1873 Kimberley was the second largest town in South Africa, having an approximate population of 40,000.
The various smaller mining companies were amalgamated by British imperialist Cecil Rhodes and Charles Rudd into De Beers, The Kimberley under Barney Barnato. In 1888, the two companies merged to form De Beers Consolidated Mines, which once had a monopoly over the world's diamond market; that monopoly had ended by 2005, following an antitrust lawsuit in the US, a voluntary agreement between De Beers and the European Commission. The latter agreement had been overturned upon appeal by the Russian mining company Alrosa, but the European Court of Justice upheld the decision and the European Commission subsequently concluded its investigation with no more action being taken against De Beers. Today, annual global rough diamond production is estimated to be about 130 million carats, of which 92% is cut and polished in India in the city of Surat; some 85% of the world's rough diamonds, 50% of cut diamonds, 40% of industrial diamonds are traded in Antwerp, Belgium—the diamond center of the world. The city of Antwerp hosts the Antwerpsche Diamantkring, created in 1929 to become the first and biggest diamond bourse dedicated to rough diamonds.
Antwerp's association with diamonds began in the late 15th century when a new techniq
Facets are flat faces on geometric shapes. The organization of occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure. Gemstones have facets cut into them in order to improve their appearance by allowing them to reflect light. Of the hundreds of facet arrangements that have been used, the most famous is 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 the late 17th century. On, the first angles for an "ideal" cut diamond were calculated by Marcel Tolkowsky in 1919. Slight modifications have been made since but angles for "ideal" cut diamonds are still similar to Tolkowsky's formula. Round brilliants cut before the advent of "ideal" angles are referred to as "Early round brilliant cut" or "Old European brilliant cut" and are considered poorly cut by today's standards, though there is still interest in them from collectors.
Other historic diamond cuts include the "Old Mine Cut", similar to early versions of the round brilliant, but has a rectangular outline, the "Rose Cut", a simple cut consisting of a flat, polished back, 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 the stone to help prevent chipping of the pavilion point. Earlier brilliant cuts have large culets, while modern brilliant cut diamonds 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 and colorful dispersion, referred to as "fire", brightly colored flashes of reflected light known as scintillation. Transparent to translucent stones are faceted, although opaque materials may be faceted as the luster of the gem will produce appealing reflections.
Pleonaste and black diamond are examples of opaque faceted gemstones. The angles used for each facet play a crucial role in the final 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 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 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", the effect caused by it is called "windowing" as the area will appear transparent and without brilliance. This is common in poorly cut commercial gemstones. Gemstones with higher refractive indexes make more desirable gemstones, the critical angle decreases as refractive indices increase, allowing for greater internal reflections as the light is less to escape.
This machine uses a motor-driven plate to hold a flat disk for the purpose of cutting or polishing. Diamond abrasives bonded to metal or resin are used for cutting laps, a wide variety of materials are used for polishing laps in conjunction with either fine diamond powder or oxide-based polishes. Water is used for cutting, while either oil or water is used for the polishing process; the machine uses a system called a "mast" which consists of an angle readout, height adjustment and a gear with a particular number of teeth 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 called a "cheater"; the stone is bonded to a rod known as a "dop" or "dop stick" and is held in place by part of the mast referred to as the "quill". The dopped stone is ground at precise angles and indexes on cutting laps of progressively finer grit, 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 accepted theory is that the fine abrasive particles of a polishing compound produce abrasions smaller than the wavelengths of light, thus making the minute scratches invisible. Since gemstones have two sides, a device 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 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 resulted; the preferred method of splitting diamonds into smaller pieces is now sawing. An older and more primitive style of faceting machine called a jamb peg machine used wooden dop sticks of precise length and a "mast" system consisting of a plate with holes placed in it.
By placing the back end of the dop into one of the many holes, the stone could
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines connecting a common point, the apex, to all of the points on a base, in a plane that does not contain the apex. Depending on the author, the base may be restricted to be a circle, any one-dimensional quadratic form in the plane, any closed one-dimensional figure, or any of the above plus all the enclosed points. If the enclosed points are included in the base, the cone is a solid object. In the case of a solid object, the boundary formed by these lines or partial lines is called the lateral surface. In the case of line segments, the cone does not extend beyond the base, while in the case of half-lines, it extends infinitely far. In the case of lines, the cone extends infinitely far in both directions from the apex, in which case it is sometimes called a double cone. Either half of a double cone on one side of the apex is called a nappe.
The axis of a cone is the straight line, passing through the apex, about which the base has a circular symmetry. In common usage in elementary geometry, cones are assumed to be right circular, where circular means that the base is a circle and right means that the axis passes through the centre of the base at right angles to its plane. If the cone is right circular the intersection of a plane with the lateral surface is a conic section. In general, the base may be any shape and the apex may lie anywhere. Contrasted with right cones are oblique cones, in which the axis passes through the centre of the base non-perpendicularly. A cone with a polygonal base is called a pyramid. Depending on the context, "cone" may mean a convex cone or a projective cone. Cones can be generalized to higher dimensions; the perimeter of the base of a cone is called the "directrix", each of the line segments between the directrix and apex is a "generatrix" or "generating line" of the lateral surface. The "base radius" of a circular cone is the radius of its base.
The aperture of a right circular cone is the maximum angle between two generatrix lines. A cone with a region including its apex cut off by a plane is called a "truncated cone". An "elliptical cone" is a cone with an elliptical base. A "generalized cone" is the surface created by the set of lines passing through a vertex and every point on a boundary; the volume V of any conic solid is one third of the product of the area of the base A B and the height h V = 1 3 A B h. In modern mathematics, this formula can be computed using calculus – it is, up to scaling, the integral ∫ x 2 d x = 1 3 x 3. Without using calculus, the formula can be proven by comparing the cone to a pyramid and applying Cavalieri's principle – comparing the cone to a right square pyramid, which forms one third of a cube; this formula cannot be proven without using such infinitesimal arguments – unlike the 2-dimensional formulae for polyhedral area, though similar to the area of the circle – and hence admitted less rigorous proofs before the advent of calculus, with the ancient Greeks using the method of exhaustion.
This is the content of Hilbert's third problem – more not all polyhedral pyramids are scissors congruent, thus volume cannot be computed purely by using a decomposition argument. The center of mass of a conic solid of uniform density lies one-quarter of the way from the center of the base to the vertex, on the straight line joining the two. For a circular cone with radius r and height h, the base is a circle of area π r 2 and so the formula for volume becomes V = 1 3 π r 2 h; the slant height of a right circular cone is the distance from any point on the circle of its base to the apex via a line segment along the surface of the cone. It is given by r 2 + h 2, where r is the radius of the base and h is the height; this can be proved by the Pythagorean theorem. The lateral surface area of a right circular cone is L S A = π r l where r is the radius of the circle at the bottom of the cone and l is the slant height of the cone; the surface area of the bottom c
The princess cut is a diamond cut shape used in engagement rings. The name dates back to the 1960s, while the princess cut as it exists was created by Betazel Ambar and Israel Itzkowitz in 1980; the cut has a square or rectangular shape when viewed from above, from the side is similar to that of an inverted pyramid with four beveled sides. Its popularity was at its highest in the 80s and 90s, though its popularity was high in the 2000s as well, it is the second most popular diamond cut, above cushion. The face-up shape of the princess cut (technical name'square modified brilliant' is square or rectangular and the profile or side-on shape is similar to that of an inverted pyramid with four beveled sides; the design is feminine. When looked down on, it bares an X shape, they are less expensive and less cut than round diamonds. The sharp points of the diamond make it more prone to damage; the number of chevrons can affect the overall outlook of a princess cut diamond. This can be determined by the wire diagram, plotted in diamond grading reports.
The princess cut had its origins in the early "French" cut. The name'princess cut' was applied in the 1960s to a cut created by Arpad Nagy called the profile cut. Following this, more square cuts were given the name; these include the barion cut and the quadrillion cut, which were precursors to the current princess cut. The princess cut was created by Betazel Ambar and Israel Itzkowitz in 1980, it is one of the newest diamond shapes. As of 2015, princess cut. 30% of engagement rings use princess cut diamonds, behind round diamonds and cushions. It saw its popularity at its peak in the 90s; the princess cut experienced a rise in popularity from the early 2000s to the mid 2000s. In the 2000s, the most popular engagement ring featured a princess cut diamond surrounded by round brilliant-cut diamonds. Disney in conjunction with Zales created a series of Disney Princess rings, with some of them, such as Aurora's, Fa Mulan's, Snow White's, Tinker Bell's featuring princess cuts. Princes cut diamonds have been used in different sports awards.
The Chicago Cubs' trophy for their 2016 World Series win featured, among others, two princess cut diamonds. In 2018, The Capitals' Stanley Cup rings featured 22 princess cut diamonds among hundreds others
A crystal or crystalline solid is a solid material whose constituents are arranged in a ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations; the scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification; the word crystal derives from the Ancient Greek word κρύσταλλος, meaning both "ice" and "rock crystal", from κρύος, "icy cold, frost". Examples of large crystals include snowflakes and table salt. Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. Examples of polycrystals include most metals, rocks and ice. A third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever.
Examples of amorphous solids include glass and many plastics. Despite the name, lead crystal, crystal glass, related products are not crystals, but rather types of glass, i.e. amorphous solids. Crystals are used in pseudoscientific practices such as crystal therapy, along with gemstones, are sometimes associated with spellwork in Wiccan beliefs and related religious movements; the scientific definition of a "crystal" is based on the microscopic arrangement of atoms inside it, called the crystal structure. A crystal is a solid where the atoms form a periodic arrangement.. Not all solids are crystals. For example, when liquid water starts freezing, the phase change begins with small ice crystals that grow until they fuse, forming a polycrystalline structure. In the final block of ice, each of the small crystals is a true crystal with a periodic arrangement of atoms, but the whole polycrystal does not have a periodic arrangement of atoms, because the periodic pattern is broken at the grain boundaries.
Most macroscopic inorganic solids are polycrystalline, including all metals, ice, etc. Solids that are neither crystalline nor polycrystalline, such as glass, are called amorphous solids called glassy, vitreous, or noncrystalline; these have no periodic order microscopically. There are distinct differences between crystalline solids and amorphous solids: most notably, the process of forming a glass does not release the latent heat of fusion, but forming a crystal does. A crystal structure is characterized by its unit cell, a small imaginary box containing one or more atoms in a specific spatial arrangement; the unit cells are stacked in three-dimensional space to form the crystal. The symmetry of a crystal is constrained by the requirement that the unit cells stack with no gaps. There are 219 possible crystal symmetries, called crystallographic space groups; these are grouped into 7 crystal systems, such as hexagonal crystal system. Crystals are recognized by their shape, consisting of flat faces with sharp angles.
These shape characteristics are not necessary for a crystal—a crystal is scientifically defined by its microscopic atomic arrangement, not its macroscopic shape—but the characteristic macroscopic shape is present and easy to see. Euhedral crystals are those with well-formed flat faces. Anhedral crystals do not because the crystal is one grain in a polycrystalline solid; the flat faces of a euhedral crystal are oriented in a specific way relative to the underlying atomic arrangement of the crystal: they are planes of low Miller index. This occurs; as a crystal grows, new atoms attach to the rougher and less stable parts of the surface, but less to the flat, stable surfaces. Therefore, the flat surfaces tend to grow larger and smoother, until the whole crystal surface consists of these plane surfaces. One of the oldest techniques in the science of crystallography consists of measuring the three-dimensional orientations of the faces of a crystal, using them to infer the underlying crystal symmetry.
A crystal's habit is its visible external shape. This is determined by the crystal structure, the specific crystal chemistry and bonding, the conditions under which the crystal formed. By volume and weight, the largest concentrations of crystals in the Earth are part of its solid bedrock. Crystals found in rocks range in size from a fraction of a millimetre to several centimetres across, although exceptionally large crystals are found; as of 1999, the world's largest known occurring crystal is a crystal of beryl from Malakialina, Madagascar, 18 m long and 3.5 m in diameter, weighing 380,000 kg. Some crystals have formed by magmatic and metamorphic processes, giving origin to large masses of crystalline rock; the vast majority of igneous rocks are formed from molten magma and the degree of crystallization depends on the conditions under which they solidified. Such rocks as granite, which have cooled slowly and under great pressures, have crystallized.
A gemstone is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks and organic materials that are not minerals are used for jewelry and are therefore considered to be gemstones as well. Most gemstones are hard, but some soft minerals are used in jewelry because of their luster or other physical properties that have aesthetic value. Rarity is another characteristic. Apart from jewelry, from earliest antiquity engraved gems and hardstone carvings, such as cups, were major luxury art forms. A gem maker is called a gemcutter; the traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between precious and semi-precious. In modern use the precious stones are diamond, ruby and emerald, with all other gemstones being semi-precious; this distinction reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, hard, with hardnesses of 8 to 10 on the Mohs scale.
Other stones are classified by their color and hardness. The traditional distinction does not reflect modern values, for example, while garnets are inexpensive, a green garnet called tsavorite can be far more valuable than a mid-quality emerald. Another unscientific term for semi-precious gemstones used in art history and archaeology is hardstone. Use of the terms'precious' and'semi-precious' in a commercial context is, misleading in that it deceptively implies certain stones are intrinsically more valuable than others, not the case. In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology; the first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon and rubies of aluminium oxide. Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is found in.
For example, which have a cubic crystal system, are found as octahedrons. Gemstones are classified into different groups and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Other examples are the emerald, red beryl, goshenite and morganite, which are all varieties of the mineral species beryl. Gems are characterized in terms of refractive index, specific gravity, cleavage and luster, they may exhibit double refraction. They may have a distinctive absorption spectrum. Material or flaws within a stone may be present as inclusions. Gemstones may be classified in terms of their "water"; this is a recognized grading of the gem's luster, transparency, or "brilliance". Transparent gems are considered "first water", while "second" or "third water" gems are those of a lesser transparency. There is no universally accepted grading system for gemstones. Diamonds are graded using a system developed by the Gemological Institute of America in the early 1950s.
All gemstones were graded using the naked eye. The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity. Other gemstones are still graded using the naked eye. A mnemonic device, the "four Cs", has been introduced to help the consumer understand the factors used to grade a diamond. With modification, these categories can be useful in understanding the grading of all gemstones; the four criteria carry different weight depending upon whether they are applied to colored gemstones or to colorless diamonds. In diamonds, cut is the primary determinant of value, followed by color. Diamonds are meant to sparkle, to break down light into its constituent rainbow colors, chop it up into bright little pieces, deliver it to the eye. In its rough crystalline form, a diamond will do none of these things. In gemstones that have color, including colored diamonds, it is the purity and beauty of that color, the primary determinant of quality. Physical characteristics that make a colored stone valuable are color, clarity to a lesser extent, unusual optical phenomena within the stone such as color zoning and asteria.
The Greeks, for example valued asteria gemstones, which were regarded as powerful love charms, Helen of Troy was known to have worn star-corundum. Aside from the diamond, the ruby, emerald and opal have been considered to be precious. Up to the discoveries of bulk amethyst in Brazil in the 19th century, amethyst was considered a precious stone as well, going back to ancient Greece. In the last century certain stones such as aquamarine and cat's eye have been popular and hence been regarded as precious. Today such a distinction is no longer made by the gemstone trade. Many gemstones are used in the most expensive jewelr