Not all diamonds that are found on Earth originated here. A type of diamond called carbonado that is found in South America and Africa may have been deposited there via an asteroid impact (not formed from the impact) over 3 billion years ago. These diamonds may have formed in the intrastellar environment, but as of 2008, there was no scientific consensus on how carbonado diamonds formed.
Presolar grains in many meteorites found on Earth contain nanodiamonds of extraterrestrial origination, probably formed in supernovas. Scientific evidence shows that white dwarf stars have a core of crystallized carbon and oxygen nuclei. The largest of these found in the whole universe so far, BPM 37093, is located 50 light-years (4.7×1014 km) away in the constellation Centaurus. A news release from the Harvard-Smithsonian Center for Astrophysics described that the 2,500-mile (4,000 km)-wide stellar core as a diamond.
Diamond and graphite are two different allotropes of carbon: pure forms of the same element that differ in structure.
Being a form of carbon, diamond oxidizes in air if it is heated over 700 °C. In the absence of oxygen, for e.g. in a flow of high-purity argon gas, diamond can be heated up to about 1700 °C. Its surface blackens, yet it can be recovered by re-polishing. At high pressure diamond can be heated up to 2500 °C, and a report published in 2009 suggests that diamond can withstand temperatures of 3000 °C and above.
Diamonds are carbon crystals that usually form deep within the Earth under high temperatures and extreme pressures. At surface air pressure, diamonds are not as stable as graphite, and so the decay of diamond is thermodynamically favorable (δH = −2 kJ / mol). So, contrary to De Beers' ad campaign extending from 1948 to at least 2006 under the slogan "A diamond is forever”, diamonds are certainly not forever. However, owing to a very large kinetic energy barrier, diamonds are generally metastable; they will not decay into graphite under normal conditions.
However, Gemstones created in lab are not imitations. For example, diamonds, ruby, sapphires and emeralds have been manufactured in labs to possess identical chemical and physical characteristics as with the naturally occurring variety. Synthetic corundums, including ruby and sapphire, are very common and they cost only comparatively lesser than the natural stones. Smaller synthetic diamonds are manufactured in large quantities as industrial abrasives. In addition larger synthetic diamonds of gemstone quality, especially of the colored variety, are also manufactured.
Some gemstones are manufactured in order to imitate other gemstones. As an example, cubic zirconia is a synthetic diamond simulant composed of zirconium oxide. Moissanite is another similar example. The imitations copy the look and color of the real ones but possess neither their chemical nor physical characteristics.
Whether a gemstone is a natural stone or a lab-created stone, the characteristics of each are similar. Lab-created stones usually tend to have a more vivid color to them, as impurities are not present in a lab, so therefore do not affect the clarity or color of the stone.
Heat can improve gemstones color or clarity. The heating process has been well known to gem miners and cutters for centuries, and in many stone types heating is commonly practiced. Most citrine is made by treating amethyst with heat and partial heating with strong gradient results in ametrine - a stone partly amethyst and partly citrine. Much aquamarine is heat treated to remove yellow tones and to change the green color into the more desirable blue or enhance its existing blue color to a purer blue. Nearly all tanzanite is heated at low temperatures to remove brown undertones and give a more suitable blue/purple color. A considerable portion of all sapphire and ruby is treated with various heat treatments to improve both color and clarity.
When jewelry containing diamonds is heated the diamond should be protected with boracic acid; else the diamond could be burned on the surface or even burned completely up. When jewelry containing sapphires or rubies is heated up, it should not be coated with boracic acid or any other substance, as this can etch the surface; it does not have to be "protected" like a diamond.
The market for industrial-grade diamonds operates very much differently from its gem-grade counterpart. Industrial diamonds are mostly valued for their hardness and heat conductivity, making many of the gemological characteristics of diamonds, such as clarity and color, irrelevant for most applications. This helps explain why 80% of mined diamonds are unsuitable for use as gemstones, are destined for industrial use. In addition to mined diamonds, synthetic diamonds found industrial applications almost immediately after their invention in the 1950s; another 570 million carats (114 tons) of synthetic diamond is produced annually for the industrial purpose. Approximately 90% of diamond grinding grit is basically of synthetic origin.
The boundary between gem-quality diamonds and industrial diamonds is partly defined and partly depends on market conditions. Within the category of industrial diamonds, there is a sub-category comprising the lowest-quality, mostly opaque stones, which are called as bort.
Industrial use of diamonds has been historically 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 the other diamonds. Common industrial adaptations of this typical ability include diamond-tipped drill bits and saws, and the use of diamond powder as an abrasive. Less expensive industrial-grade diamonds, known as bort, with more flaws and poorer color than other gems, are used for such purposes. Diamond is not suitable for machining ferrous alloys at high speeds, as carbon is soluble in iron at the higher temperatures created by high-speed machining, leading to largely increased wear on diamond tools when compared to other alternatives.
Specialized applications involve use in laboratories as containment for high pressure experiments, high-performance bearings, and limited use in specialized windows. With the continuing advances being made in the production of synthetic diamonds, future applications have become feasible. Garnering much excitement is the main use of diamond as a semiconductor suitable to build microchips, or the use of diamond as a heat sink in electronics.
Diamond clarity is a quality of diamonds related to the existence and visual appearance of internal characteristics of a diamond called inclusions, and surface defects called blemishes. Clarity is the important one of the four Cs of diamond grading, the others being carat, color, and cut. Inclusions may either be crystals of a foreign material or another diamond crystal, or structural imperfections such as tiny cracks that can appear whitish or cloudy. Factors such as the number, size, color, relative location, orientation, and visibility of inclusions can affect the relative clarity of a diamond. A clarity grade is assigned based on the overall appearance of the stone less than 10x magnification.
Most inclusions that are present in gem-quality diamonds don't affect the diamonds' performance or structural integrity. However, large clouds can in turn affect a diamond's ability to transmit and scatter light. Large cracks that are close to or breaking the surface may reduce a diamond's resistance to fracture.
Usually diamonds with higher clarity grades are more valued, with the exceedingly rare "flawless" graded diamond fetching the highest price. Minor inclusions or blemishes are useful, as they can also be used as unique identifying marks analogous to fingerprints. In addition, as synthetic diamond technology improves and distinguishing between natural and synthetic diamonds becomes very difficult, inclusions or blemishes can be used as proof of natural origin.
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Diamonds form between 120-200 kms or 75-120 miles deep in the earth's surface. According to geologists the first delivery of diamonds was somewhere around 2.5 billion years ago and the latest was 45 million years ago. According to science, the carbon that makes diamonds comes from the melting of pre-existing rocks in the Earth's upper surface mantle. There is an abundant quantity of carbon atoms in the mantle.
Temperature changes in the upper mantle forces the carbon atoms to go deeper and it melts and finally becomes new rock, when the temperature reduces. If other conditions like pressure and chemistry works right then the carbon atoms in the melting crustal rock bond to build diamond crystals. Yet there is no guarantee that these carbon atoms will surely turn into diamonds. Either if the temperature rises or the pressure drops then the diamond crystals may melt partially or totally dissolve. Even if they do form, it would take thousands of years for those diamonds to come anywhere near the surface.