Diamonds

Diamond is the hardest known natural material (third-hardest known material after aggregated diamond nanorods and ultra hard fullerite), whose hardness and high dispersion of light make it useful for industrial applications and jewelry.

Diamonds are specifically renowned as a material with superlative physical qualities — they make excellent abrasives because they can be scratched only by other diamonds, Borazon, ultra hard fullerite, or aggregated diamond nanorods, which also means they hold a polish extremely well and retain their luster. About 130 million carats (26,000 kg) are mined annually, with a total value of nearly USD $9 billion. About 100,000 kg are synthesized annually.

The name “diamond” derives from the ancient Greek adamas (αδάμας; “invincible”). They have been treasured as gemstones since their use as religious icons in India at least 2,500 years ago— and usage in drill bits and engraving tools also dates to early human history. Popularity of diamonds has risen since the 19th century because of increased supply, improved cutting and polishing techniques, growth in the world economy, and innovative and successful advertising campaigns. They are commonly judged by the “four Cs”: carat, clarity, color, and cut.

Roughly 49% of diamonds originate from central and southern Africa, although significant sources of the mineral have been discovered in Canada, India, Russia, Brazil, and Australia. They are mined from kimberlite and lamproite volcanic pipes, which brought to the surface the diamond crystals from deep in the Earth where the high pressure and temperature enables the formation of the crystals. The mining and distribution of natural diamonds are subjects of frequent controversy such as with concerns over the sale of conflict diamonds by African paramilitary groups. There are also allegations that the De Beers Group misuses its dominance in the industry to control supply and manipulate price via monopolistic practices, although in recent years the company's market share has dropped to below 50%.

Material properties

A diamond is a transparent crystal of tetrahedrally bonded carbon atoms. Diamonds have been adapted for many uses because of the material's exceptional physical characteristics. Most notable are its extreme hardness, its high dispersion index, and high thermal conductivity.

Hardness

Diamond is the hardest natural material known, its hardness set to 10, i.e. hardest, on Mohs scale of mineral hardness and having an absolute hardness value of between 90, 167, and 231 gigapascals in various tests. Diamond's hardness has been known since antiquity, and is the source of its name. However, aggregated diamond nanorods, an allotrope of carbon first synthesized in 2005, are now believed to be even harder than diamond.

The hardest diamonds in the world are from the New England area in New South Wales, Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is considered to be a product of the crystal growth form, which is single stage growth crystal. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice all of which affect their hardness.

The hardness of diamonds contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well, keeping its luster over long periods of time. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in an engagement ring or wedding ring, which are often worn every day.

Industrial use of diamonds has historically been associated with their hardness; this property makes diamond the ideal material for cutting and grinding tools. As the hardest known naturally occurring material, diamond can be used to polish, cut, or wear away any material, including other diamonds. Common industrial adaptations of this ability include diamond-tipped drill bits and saws, or use of diamond powder as an abrasive. Industrial-grade diamonds are either unsuitable for use as gems or synthetically produced, which lowers their value and makes their use economically feasible. Industrial applications, especially as drill bits and engraving tools, also date to ancient times.

Electrical conductivity

Other specialized applications also exist or are being developed, including use as semiconductors: some blue diamonds are natural semiconductors, in contrast to most other diamonds, which are excellent electrical insulators.

Toughness

Toughness relates to a material's ability to resist breakage from forceful impact. The toughness of natural diamond has been measured as 3.4 MPa\sqrt{m} , which is good compared to other gemstones, but poor compared to most engineering materials. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond is therefore more fragile in some orientations than others.

Color

Diamonds can occur in nearly any color, though yellow and brown are by far the most common. "Black" diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. When the color is saturated enough in yellow or brown diamonds, a stone may be referred to as a fancy colored diamond by the gem trade, otherwise they are graded for color in the normal color range of white diamonds. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the crystal lattice, known as a carbon flaw. The most common impurity, nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present. The Gemological Institute of America (GIA) classifies low saturation yellow and brown diamonds as diamonds in the normal color range, and applies a grading scale from 'D' (colorless) to 'Z' (light yellow). The GIA labels diamonds that have more color than a 'Z' diamond fancy, along with those that are any color other than yellow or brown.

Natural history

Formation

Diamonds are formed in numerous different ways. One way they are formed is by prolonged exposure of carbon-bearing materials to high pressure at comparatively low temperature. On Earth, the formation of diamonds is possible because there are regions deep within the Earth that are at a high enough pressure and low enough temperature that the formation of diamonds is thermodynamically favorable. Beneath thick continental crust ( in the lithospheric mantle of cratons), diamonds form starting at depths of about 150 kilometers (90 miles), where pressure is roughly 5 gigapascals and the temperature is around 1200 degrees Celsius (2200 degrees Fahrenheit). Diamonds rise to the Earth surfaces carried by the molten rock of kimberlite or lamproite diatremes. Diamonds cannot form beneath oceanic crust because the oceanic mantle is too hot at the appropriate depth. Thick continental lithosphere is cooler than asthenosphere at the same depth, reaching the required pressure at a low enough temperature for diamond stability. Long residence in the cratonic lithosphere allows diamond crystals to grow larger.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Diamonds.

Diamond -- History

Diamonds are thought to have been first recognized and mined in India, where significant alluvial deposits of the stone could then be found. The earliest written reference can be found in the Buddhist text, the Anguttara Nikaya another sanskrit text, the Arthashastra, which was completed around 296 BCE and describes diamond's hardness, luster, and dispersion. Diamonds quickly became associated with divinity, being used to decorate religious icons, and were believed to bring good fortune to those who carried them. Ownership was restricted among various castes by color, with only kings being allowed to own all colors of diamond.

In February 2005, a joint Chinese-U.S. team of archaeologists reported the discovery of four corundum-rich stone ceremonial burial axes originating from China's Liangzhu and Sanxingcun cultures (4000 BCE–2500 BCE) which, because of the axes' specular surfaces, the scientists believe were polished using diamond powder [2] [3]. Although there are diamond deposits now known to exist close to the burial sites, no direct evidence of coeval diamond mining has been found: the researchers came to this conclusion by polishing corundum using various lapidary abrasives and modern techniques then comparing the results using an atomic force microscope. At that scale, the surface of the modern diamond-polished corundum closely resembled that of the axes; however, the polishes of the latter were superior.

Diamonds were traded to both the east and west of India and were recognized by various cultures for their gemological 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. In China, diamonds seem to have been used primarily as diamond tools for engraving jade and drilling holes in beads. Archaeological evidence from Yemen suggests that diamonds were used as drill tips as early as the 4th century BCE. In Europe, however, diamonds disappeared for almost 1,000 years following the rise of Christianity because of two effects: early Christians rejected diamonds because of their earlier use in amulets, and Arabic traders restricted the flow of trade between Europe and India.


Until the late Middle Ages, diamonds were most prized in their natural octahedral state, perhaps with the crystal surfaces polished to increase luster and remove foreign material. Around 1300, the flow of diamonds into Europe increased via Venice's trade network, with most flowing through the low country ports of Bruges, Antwerp, and Amsterdam. During this time, the taboo against cutting diamonds into gem shapes, which was established over 1,000 years earlier in the traditions of India, ended allowing the development of diamond cutting technology to begin in earnest. By 1375, a guild of diamond polishers had been established at Nuremberg. Over the following centuries, various diamond cuts were introduced which increasingly demonstrated the fire and brilliance that makes diamonds treasured today: the table cut, the briolette (around 1476), the rose cut (mid-16th century), and by the mid-17th century, the Mazarin, the first brilliant cut diamond design. In 1919, Marcel Tolkowsky developed an ideal round brilliant cut design that has set the standard for comparison of modern gems; however, diamond cuts have continued to be refined.

The rise in popularity of diamonds as gems seems to have paralleled increasing availability through European history. In the 13th century, King Louis IX of France established a law that only the king could own diamonds. However, within a century diamonds were popular gems among the moneyed aristocratic and merchant classes, and by 1477 had begun to be used in wedding rings. Popularity continued to rise as new cuts were developed that enhanced the diamond's aesthetic appeal, and has largely continued unabated to this day; diamonds have proven popular with all classes in society as their cost has become within reach. A number of large diamonds have become historically significant objects, as their inclusion in various sets of crown jewels and the purchase, sale, and sometimes theft of notable diamonds, have sometimes become politicized.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Diamonds.

Jewelry: Karat - Carat

Karat

A unit of measure for the fineness of gold, equal to 1/24 part. Pure gold is 24 karat; gold that is 50 percent pure is 12 karat.

Carat (mass)

A unit of weight for precious stones, equal to 200 milligrams.

Note: A carat can also be further divided into "points". There are 100 points to a carat.

The carat is a unit of mass used for gems, and equals 200 milligrams. The word derives from the Greek keration (fruit of the carob), via Arabic and Italian. Carob seeds were used as weights on precision scales because of their uniform size. In the past, different countries each had their own carat, roughly equivalent to a carob seed. However, in 1907 the metric carat of 200 milligrams was adopted, which is now universally used today.

Carat

The carat is a measure of the purity of precious metals and their alloys, such as gold. One carat in this sense is one twenty-fourth purity by weight. Therefore 24-carat gold is pure gold, 12-carat gold is 50% purity, etc. In the United States and Canada, the spelling karat is usually used for the measure of purity, while carat refers to the measure of mass.

The carat system is increasingly being complemented or superseded by the millesimal fineness system in which the purity is denoted by parts per thousand of pure metal in the alloy.

The most common carats used for gold in bullion, jewellry making and goldsmith are:

• 24 carat (millesimal fineness 999)

• 22 carat (millesimal fineness 916)

• 20 carat (millesimal fineness 833)

• 18 carat (millesimal fineness 750)

• 16 carat (millesimal fineness 625)

• 14 carat (millesimal fineness 585)

• 10 carat (millesimal fineness 417)

• 9 carat (millesimal fineness 375)