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Diamond
 Diamond, throughout history, has held a pre-eminent place among gemstones. It is certainly the most precious of all nature's creations. But most people know very little about diamonds. Diamond is pure carbon, formed under extreme pressure and heat at our Earth's mantle at depths exceeding 100 miles from the surface. This temperature must have been maintained for a very long period during or after which the magma containing the diamonds was forced up to the surface, picking up other minerals on the way and forming the 'kimberlite pipes' from which all diamonds ultimately come. They reach the surface in clusters. Diamonds are only found within a pipe, never so much as a foot beyond it in the surrounding rock.  Formation of Kimberlite Pipes The oldest source of Diamonds is India, where they were known centuries before the Christian era. Brazil was the main producer during the nineteenth century but the mines were exploited so fast that their yield decreased even before the 1980s brought South Africa to the fore. The most important recent discoveries have been in the northern Siberia, and other places further north beyond the Arctic Circle. Russians claim that their production exceeds that of South Africa. Europe has no diamonds but small number of stones have been found in widely scattered localities in the USA. Most of the diamonds mined are translucent to opaque and vary in color from grey to brown. Such stones are useless as gems but they are of the utmost importance in the industry. They are used as tips for metal shaping tools, as dies for drawing wire, as abrasives for truing grinding wheels, for drill bits and for other uses. Clarity Gem diamonds, however, should be clearly transparent to be worth cutting. Even small inclusions or imperfections in the cutting of the stone, invisible to the naked eye but visible through a loupe, detract from the value of a stone. The grades used for evaluating clarity are: F (Flawless ):Shows no inclusions or blemishes of any sort under 10x magnification when observed by an experienced grader. VVS: Very very slightly included. Contains minute inclusions that are difficult even for experienced grader to see under 10x magnification. VS: Very Slightly included. Contains minute inclusions such as crystals, clouds or feathers when observed with effort under 10x magnification. SI1, SI2 : Slightly Included. Contains inclusions (clouds, included crystals, knots, cavities and feathers) that are noticeable to an experienced grader under 10x magnification. I1, I2, I3 :Included. Contains inclusions (possibly large feathers or large included crystals) that are obvious under 10x magnification and may affect transparency and brilliance. To popular belief, higher clarity does not necessarily mean more beautiful. If the inclusions are not visible to the naked eye, a higher clarity does not really improve the appearance of a diamond but rather the rarity and price. A higher clarity is more desirable and valuable, but knowing that you have selected the right clarity for the right reasons is most important.  ColorDiamonds should be completely colorless, but if colored, the color must be definite and attractive. Brown and grey colors are not esteemed highly, nor is black, which, contrary to popular belief, is not very rare. More highly valued colors are green, canary yellow, blue or pink. Well-colored stones are rare; they tend to be associated with certain mines. Most colorless diamonds have in fact a faint tingle of yellow or brown, discernible only to the practiced eye. Some fluoresce blue so strongly, even in daylight, that any yellow tingle is compensated. Completely colorless 'blue-white' diamonds are very rare.In the USA a system of letters is used by the Gemological Institute of America.  The first five grades would appear quite colorless to the untrained eye. Only an expert, working in a neutral north light and with the aid of standard stones for comparison, can come to a reliable assessment. Cut The value of a diamond is also influenced by it's 'make'. In cutting a stone it is essential not only that the facets should be placed symmetrically and be of the same size and well polished, but also that the general shape of the stone should conform to certain proportions. To save labor or material, stones are sometimes made too deep or too shallow with a consequent loss of brilliance. The ideal proportions have been calculated theoretically and assessed in practice.
Most modern brilliants come very close to these requirements, but those cut sixty years or more ago tend to be very deep. Unless deviations from the ideal are small, it is seldom advisable to have such stones re-cut as the loss in weight would depreciate them more than the improvement in quality would warrant; the value of a diamond increases roughly as the square of the weight, so that of two stones of the same quality but of different size, the one four times greater in weight would be worth approximately sixteen times as much. A loss in weight would therefore depreciate a stone more than proportionately. The greater rarity of larger stones, the greater care needed in fashioning them and the greater loss of rough diamond involved account for the difference in values. More than half the raw material is lost in cutting a diamond in any case, but in larger stones losses are often greater. There is a great deal of cuts. Most of the standard cuts are listed bellow.
The value of a diamond therefore depends on the clarity, color, cut and carat weight of the finished stone, the 'four Cs', but the importance allotted to each of these factors may well very from stone to stone and valuation is not a matter of rule of thumb reckoning. Fashion and the condition of the market for stones of a particular kind at a particular time also have an important influence. 
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