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This explores typical gemstone cuts and faceting, as well as providing a tabular presentation of the relationship of many common gemstones to crystal systems and classes. The Annex also shows the common association of gemstones with the chakra system.
The proportions of a stone, as well as its polish and precision of faceting, determine how much of the diamond's potential fire and beauty may be released. The way a diamond is cut profoundly influence its sparkle, fire and brilliance, as well as its perceived size and even, to some degree its apparent color. In order to maximize the diamond's brilliance it must be well polished and cut in a geometrically precise manner. This means properly aligning the facets so light will enter the diamond and reflect back through the large top facet, or table of the diamond.
As a general rule only translucent, semi-translucent or clear gemstones are cut with facets. There are several basic cuts:
There are many other types of cuts of gemstones, cushion top, rose cut, pear cut, fancy cut, etc. These cuts are relatively rare compared to the six cuts described above. Bastard-cut a term used for fashioned stones which do not conform to the recognised typical forms, or which show some slight modification from the "pure" forms. The term applies only to those stones which have a regular and symmetrical arrangement of the facets; should they be irregular or haphazard the term Cap-cut is used.
There is no universal agreement on what angles to use for faceting various gemstones. Each set of faceting angles from generally accepted sources may well have slightly different optical characteristics. The definition of which is "best" is purely a matter of personal preference.
Each facet of a brilliant has a name of its own: skew, skill, bezel, quoin, etc. facet angles [more]
(6 two-fold, 4 three-fold, 3 four-fold)
|silicate||garnet (almandine, andradite, grossular, pyrope, spessartine, uvarovite)||1||1|
|Tetragonal||3||right angles||2 horizontal axes of equal length; vertical axis is either longer or shorter.||5
(4 two-fold, 1 four-fold)
|Hexagonal||4||3 horizontal axes of equal length and at 60 degrees to each other.||vertical axis is usually longer and at right angles to the horizontal axes.||7
(6 two-fold, 1 six-fold)
|Trigonal||4||3 horizontal axes of equal length and at 60 degree angles to each other.||vertical axis is usually longer and at right angles to the horizontal axes.||4
(3 two-fold, 1 three-fold)
|3||1||oxide||corundum (ruby)||1||1, 2|
|silicate||tourmaline (buergerite, dravite, elbaite, schorl, uvite)||1||3|
|Orthorhombic||3||all at right angles to each other.||all of unequal length||3
|Monoclinic||3||2 axes are inclined at an angle other than 90 degrees||third axis is at right angles to the other two.||1
|Triclinic||3||all of unequal length (least symmetrical of all crystal systems)||all inclined at angles other than 90 degrees to each other.||0||0||1||silicate||microcline (amazonite, perthite)||5|
It was only in this century that stones were classified into various categories such as precious stones, semiprecious stones and ornamental stones. In 1880, for example, the diamond, corundum, ruby, emerald, sapphire, amethyst, agate, aventurine, garnet, lapis lazuli, opal, topaz and turquoise were all classified as precious stones. Well into the 20th century the preference was to divide the minerals used in jewelry into two categories, gems (when mounted in jewelry) and ornamental stones (carvings, mosaics, inlays, etc); the term precious stone is strictly reserved for diamonds, rubies, sapphires and emeralds. Modern classification separates minerals into several categories [more]: pure elements; sulphides; halides; oxides; carbonates; phosphates; silicates; non-crystalline and organic materials
Classification of gemstones http://www.tradeshop.com/gems/classify.html
Indian classification of gems and jewels http://www.urday.com/gemo2.htm
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