What is a LapidaryLapidary
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A lapidary (the word means "concerned with stones") is an artisan who practices the craft of working, forming and finishing stone, mineral, gemstones, and other suitably durable materials (amber, shell, jet, pearl, copal, coral, horn and bone, glass and other synthetics) into functional and/or decorative, even wearable, items (e.g. cameos, cabochons, and more complex facetted designs). The adjectival term is also extended to refer to such arts. Diamond cutters are generally not referred to as lapidaries, due to their highly specialized techniques which are required to work diamond successfully. The arts of a sculptor or stonemason are generally too broad in scale to fall within the definition, though chiseling inscriptions in stone, and preparing laboratory 'thin sections' may be considered lapidary arts. The term is most commonly associated with jewelry and decorative household items (e.g. bookends, clock faces, ornaments, etc.) A specialized form of lapidary work is the inlaying of marble and gemstones into a marble matrix, known in English as "pietra dura" for the hard stones like onyx, jasper and carnelian that are used, but called in Florence and Naples, where the technique was developed in the 16th century, opere di commessi. The Medici Chapel at San Lorenzo in Florence is completely veneered with inlaid hard stones. A lapidary specialty developed from the late 18th century in Naples and Rome are the "micro-mosaics" assembled out of many minute slivers of stone to create still life, cityscape views and the like. In China lapidary work specializing in jade carving has been continuous since the Shang dynasty. There exists three broad categories of lapidary arts. These are the procedures of tumbling, cabochon cutting, and faceting. The distinction is somewhat loose, and leaves a broad range within the term cabochon. At present most lapidary work is accomplished using motorized equipment and resin or metal bonded diamond tooling in successively decreasing particle sizes until a polish is achieved. Often, the final polish will use a different medium, such as tin oxide, glasitite or cerium(IV) oxide. Older techniques, still popular with hobbyists, used bonded grinding wheels of silicon carbide, with only using a diamond tipped saw. Diamond cutting, because of the extreme hardness of diamonds, cannot be done with silicon carbide, and requires the use of diamond tools. There are also many other forms of lapidary, not just cutting and polishing stones and gemstones. These include; casting, faceting, carving, jewelery, mosaics (eg. little slices of opal on potch, obsidian or another black stone and with a clear dome (glass or crystal quartz) on top. There are lapidary clubs through-out the world and in Australia there is an annual gemshow, the Gemborree which is a nation-wide lapidary competition. The largest mineral show in the world is the Tucson Gem and Mineral Society Show held annually in Arizona, USA.
Tumble polishing
Tumble polishing, or tumbling, is a technique for smoothing and polishing a hard substance. Within the field of metalwork, this is known under the term of barrelling, or barrel polishing, and is subtly different, but operated under the same principles. For tumbling of rocks, as a lapidary technique, a plastic or rubber barrel is loaded with consignment of rocks, all of similar, or the same hardness, and some abrasive grit and a lubricant. Silicon carbide grit is commonly used here, and water is the universal lubricant. The barrel is then placed on slowly rotating rails so that it turns. This causes the load of rock to slide past each other, with the abrasive grit between the rock. The result of this depends on the coarseness of the abrasive, and the duration of the tumble. Typically, a full tumble polish from rough rock to polish takes around 4-5 weeks, and is done in 4 steps. Initially smoothing is done with a coarse grit (such as 80 mesh), followed by washing and use of finer grits (220 then 400 mesh) before the use of a polishing compound (such as cerium oxide or ferric oxide). The precise time is determined by many factors, including the hardness of the rock, and the degree of smoothing desired in the coarser steps. During the 1970s, small rock tumblers were a common hobby item, and jewelry decorated with tumbled semi-precious stones was very much in fashion. Likewise, dishes and decorative glass jars filled with tumbled stones (often including common rocks not suitable even for costume jewelry) were frequently used as household ornaments. For barreling metals, a barrel with internal veins, typically rubber, is used. The work parts are placed in the barrel, along with polished steel shot, and a solution of a barreling soap. The soap acts as a lubricant, and to prevent the rusting of the metals, whilst the material is tumbled as for rocks. Only one stage is used, although the length of time determines the final polishing. Stained glass shards used for mosaic glass are also tumbled. No abrasive is used to avoid clouding the glass, only water as a lubricant to remove the sharp edges so that the glass may be handled safely. As little as 8 hours tumbling may be sufficient for tumbled glass. These techniques, although they take a long time, involve very little operator intervention, and thus are very cheap. Small tumblers (one pound capacity) are available and inexpensive for home/hobbyist use. Additionally, very large barrels can be used, allowing a lot of work to be done at once for professional use. The main disadvantage is its limited scope - stones will be smooth, and have semi-random shapes (like pebbles from the beach), and metals need to be relatively simple shapes, with no fine work. Sometimes, stone preforms are used. This refers to cutting shapes from the rough rock, before tumbling. This gives more control over the final piece, so shapes such as a tear drop could be produced. It is still limited to rounded shapes. Preforms may use less time with the coarsest step, or skip it altogether. Tumbling is used to polish and smooth dice for recreational use, this has the unfortunate effect of making their sides and faces somewhat uneven and making the dice less than fair. Lou Zocchi's Gamescience company is known for making dice without tumbling them; the edges as a result of these dice are very, very sharp but the dice are guaranteed to be fair.
Cabochon
A cabochon or cabouchon is a gemstone which has been shaped and polished as opposed to facetted. The resulting form is usually a convex top with a flat back, although the term is used to mean any deliberate shape that is not facetted. Cutting "en cabochon" is usually applied to opaque gems, while facetting is usually applied to transparent stones. Hardness is also taken into account as softer gemstones with a hardness lower than 7 on the Mohs hardness scale are easily scratched, mainly by silicon dioxide in dust and grit. This would quickly make translucent gems unattractive, whereas polished as cabochons, the scratches would not be as easily apparent. The one notable exception to this is the case of asteriated stones such as star sapphires and chatoyant stones such as cat's eye chrysoberyl, where a domed cabochon cut is used to show the star or eye, which would not be seen in a facetted cut. The usual shape for cutting cabochons is a rounded oval. This is at least partly because the eye is less sensitive to small asymmetricalities in an oval, as opposed to round shape, and because the rounded cross section fits well with the domed top. The normal proceduce is to cut a slab of the rough rock, then to stencil a shape from a template. Normally, an aluminium scriber is used to mark the rock, as it tends not to rub off in handling. The slab is then trimmed to near the marked line, followed by nibbling to the line. The purpose of nibbling is to speed the shaping of the material. It arises because the saws are diamond tipped blades, whilst most grinding is done with silicon carbide grindwheels. The diamond cuts quicker and cleaner, and the saw does not wear as fast as the grind wheel. Once the piece is trimmed and nibbled, it is usually mounted on a handle to assist manipulation. This procedure is called dopping: It is normally done by adhering the stone with wax onto a length of wooden dowel called a dop stick. The piece is then ground to the line, the edges are bevelled, and finally the top is smoothed to a uniform dome.
FacetFacets are flat faces on geometric shapes. The organization of naturally occurring facets was key to early developments in crystallography, since they reflect the underlying symmetry of the crystal structure. Gemstones commonly have facets cut into them in order to improve their appearance. Of the many hundreds of facet arrangements that have been used, the most famous is probably the round brilliant cut, used for diamond and many colored gemstones. This arrangement of 57 facets was calculated by Marcel Tolkowsky in 1919. It slight improvements have been made since then, including the addition of a 58th facet (a culet) on the bottom of the stone. Since this is calculated to show maximum brilliance, round diamonds are rarely cut in any other arrangement, although recently the Princess cut is becoming popular. Other cuts, including "rose" cuts, are most typically found in antique jewelry. See diamond cuts for an in-depth discussion and diagrams of various shapes and ways of cutting faceted stones. Cutting facets
The art of cutting a gem with facets is a very precise activity. The aim with a facetted cut is to produce an article that sparkles with internally reflected light, and that shows off the "fire" of the stone. Accordinally, normally only transparent or translucent stones are faceted. The angles between each facet are precisely calculated. As the aim is to maximise the effect of the internal reflections, these angles depend on the refractive index of the material. This means that although the name and general shape of a particular cut may be the same between different materials, the actual angles will be slightly different, for the maximum effect. Thus, although cubic zirconia and rock crystal may look similar to diamond, and all can be cut in a round brilliant cut, the angles must be different to produce the same optical effects. Additionally, as diamond has a refractive index significantly higher than the other natural transparent stones, it can have a much greater sparkle than other materials. While some facets can be cut by cleavage, specialised machines are used for cutting arbitrary facets. These consist of two main features: a flat abrasive, usually diamond dust of precise size bonded onto a metal disk (called 'laps') or carried by an oily fluid on a smooth metal or ceramic disk, and a system for holding a stone onto the disk at a precise angle and position. This usually requires the stone to be attached to a holder, which is then placed in an indexed vice. This allows progressively finer abrasives to be used without disrupting the orientation of the stone. The final abrasive must be smaller than the wavelength of light, so that the scratches it creates are invisible. Modern machines tend to have indexed gears for moving the stone, so that rotating the stone to cut the next facet can be more precisely controlled. Much less commonly, faceters use cylindrical machines, which leave concave facets. This technique is most noticeably used around the gem's girdle.