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Gem engraving, setting, and cutting
The most ancient technique of stone engraving, intaglio-incised carving, was probably first used to produce seals. The art is believed to have originated in southern Mesopotamia and was highly developed by the 4th millennium BCE. During the Hellenistic Age (c. 323–30 BCE) intaglio surface engraving gave rise to the idea of carving stones in relief, exploiting the different coloured layers of certain minerals to create contrasting figures (cameo): the background was cut down to the lower level, of a different colour or shade, in order to make the subject stand out chromatically. The stones that have properties suited for this purpose are sardonyx, agate, and onyx. The cameo is usually one of the components for necklaces, bracelets, and rings or is included in medallions with a jeweled frame. The art of cameo in jewelry was most highly developed during three periods: the late republican to early imperial period in Rome, the Renaissance, and the Neoclassical period in the 18th century. The evolution of techniques of setting has followed that of stonecutting. The insertion of gems in jewelry can be done in various ways. The setting can have a round, square, oval, or rectangular collet (rim); in periods in which gems were mounted in their own irregular shapes, the collet followed this form.
Usually, on the inside of the collet a short distance from the edge, there is a protrusion on which the stone rests. The edge is pounded down around the gem to ensure its stability. In coronet settings the form may be conical or pyramidal, solid or pierced. The edge is first shaped into a row of teeth, which are later hammered down onto the gem in order to hold it in place. Until fairly recently, nearly all gems were mounted on a metal base; and transparent stones, according to their colour, were placed on a gold or silver base to increase the amount of light reflected. As new cuts were developed for stones, setting techniques also progressed, especially for those jewels in which important stones like diamonds, emeralds, and rubies form the main theme. The tendency was to leave the stones as visible as possible (especially in rivière necklaces and bracelets made only of diamonds) by mounting them with a very small ring of white gold or platinum fitted closely against the back of the stone. Three claws, attached to this ring, hold the stone in place. Pearls, like some coloured stones, in ancient Classical times were pierced with a drill, the hole going half or all the way through according to whether the pearls were to be strung on a necklace or fastened onto a jewel.
Until the 15th century, stones were only polished or the part to be left visible was rounded into a dome shape called cabochon. The cutting known as faceting gradually developed from the first attempts in the 15th century, probably in France and the Netherlands. During the 16th century the simple rose cut began to be used, after which there were no new developments until 1640, when, under the patronage of Jules Cardinal Mazarin, the first brilliant cut was carried out (also called the Mazarin cut). Toward the end of that century, a Venitian succeeded in obtaining the triple brilliant cut, which is still used. The numerous cuts used for diamonds today are usually applied to other precious and semiprecious transparent stones as well. For emeralds, rubies, and other coloured stones the square or rectangular cut with a stepped bulb or the cabochon cut are usually used.While it would be wonderful if gemstones came out of the ground ready to wear, this typically just doesn't happen. Raw crystals from the earth are usually called 'rough gemstones' (or just 'rough') and this is actually very apt - rough gems can look pretty rough!
The different parts of a gemstone. Due to the spherical nature of most rough, the oval is the most common shape for coloured gemstones as it typically best balances beauty and weight retention. Lapidary (gem cutting) is thousands of years old and is the transformation of raw crystals into dazzling gemstones. It is the art of making the gem assume a certain shape, unlocking its lustre, colour and brilliance. Lapidaries, also known as gem cutters, have two general styles they can choose when cutting gemstones:Faceted Gems:Gems with geometrically-shaped, flat polished faces. Today, faceted gemstones are the most popular style, but this was not always the case. Big fans of cabochons, cameos (a gem carved in positive relief) and intaglios (a gem carved in negative relief), did you know that ancient Romans considered wearing faceted gems vulgar?Non-faceted: Gems that don't have geometrically-shaped, flat polished faces, such as cabochons. Derived from the old Norman French word 'caboche', meaning head, cabochon is an ancient shaping and polishing technique that remains popular today due to the yesteryear charm and character of what are typically, richly coloured gems.
Because of their different optical properties, coloured gems do not have an ideal 'brilliant cut' like Diamonds (see the diagram in this section). Which style, cut and shape lapidaries select depends on the type, shape and quality of the rough gemstone. The cut of a gem directly affects its overall value as the cut determines how well a gem returns its body colour back to the eye. The lapidary performs a juggling act between beauty and commercial considerations, such as carat weight retention. For every gem, the lapidary is looking for the best compromise between appearance and size, remembering that the value of the finished gem also depends on its carat weight. Maintaining a gem's critical angle (the maximum angle of refraction) often unavoidably results in a smaller gem. It is important to consider windows (areas of washed out colour in a table-up gem, often due to a shallow pavilion) or extinctions (areas of darkness in a table-up gem, all gemstones possess some degree of extinction) on a finished gemstone's beauty. Sometimes size does matter and big can be beautiful, but this isn't always the case. Beauty will sometimes be sacrificed to minimise rough weight loss and vice versa.
How do you tell a good cut from a bad one? No one cut is always more beautiful than another, it's all down to the magic of nature and the artistry of the lapidary. One thing that can confuse is shape versus cut. Sometimes they mean the same thing (a 'princess cut' is always square in shape) and sometimes they don't (a 'step cut' can be square, rectangular or octagon). The cut is not just a gem's shape, it is also the cutting techniques (facet arrangements, finish and proportions) used to finish the gem from the rough. A gem's shape will affect the overall look of the gem, but if it's faceted properly, the shape won't necessarily affect its value.
A diagram of some of the classic shapes and cuts are included in this section. Due to the spherical nature of most rough, the oval is the most common shape for coloured gemstones, as it typically best balances beauty and weight retention. Once you've established your shape preference, simply use the following checklist:
- The steps in cutting are slicing, pre-forming, shaping and polishing.
- First, a Diamond-tipped circular steel saw is used to slice the rough gem into pieces.
- Then it is pre-formed using a vertical steel grinding wheel.
- The shaper then uses a hand-operated shaping wheel to more accurately present its facets and size.
- The final step is polishing the gem to reveal its hidden lustre and brilliance.
- Even, uniform colour with no distinct zoning, unless of cause this is a feature of the gemstone as in bicolour varieties.
- Remember, gems are designed to be viewed from the table down. Balance, symmetry and proportion.
- Acceptable crown height and pavilion depth. The crown is usually one-half to one-third the pavilion depth.
- Acceptable brilliance, remembering that brilliance varies amongst different gemstone varieties.
- Acceptable clarity (amount and location of inclusions), again remembering that acceptable inclusions vary among the different gem varieties.
- A good polish condition, with no eye-visible scratches or polishing marks.
- Acceptable pavilion bulge and girdle thickness. Acceptable sharpness of the facet junctions.
- The majority of the gem's weight is visible from the top.
The diagram on the side shows gemstone cuts from antiquity to the present. True 'artists in stone', lapidary has evolved over thousands of years. During antiquity, cabochons were initially the only cutting style available. Guided by the natural facets of the gemstone's crystal structure, the lapidary cut gems in increasingly more complex ways over time. The earliest of these involved removing the tops of crystals. In the last hundred years, technological advances have allowed cutters to develop some breathtaking innovations.