Stressing Stones

It?s a universal truth in jewelry manufacturing: If you set gemstones, eventually one will break. A number of factors contribute to stone breakage, including species, setting styles, shapes, natural defects, and the skill of the setter. But all stone breakage can be attributed to two general causes: thermal stress and physical trauma.

Thermal stress occurs whenever the temperature within a stone changes quickly. Most stone species cannot withstand a rapid change between expansion and contraction, and they break from the resulting stress. Such fractures usually occur when the stone is heated, either deliberately or accidentally, and then quenched in water or a pickle pot.

The best way to avoid thermal shock is to avoid heating the gemstone. When re-tipping a ring, for example, never put heat directly on the stone. (Even diamonds, which are regarded as thermally stable, can break if they have internal defects.) If you can?t avoid it?sometimes heat travels through a ring shank during soldering?allow the piece to air cool to room temperature before pickling. Pay special attention to opal and pearl. Because it usually contains natural moisture, opal should never be heated above 180°F. Pearls, too, are sensitive. Whenever possible, remove them before heating, or at least protect them with a heat-shielding material.

Other sources of potentially damaging heat exposure include steam cleaning machines, ultrasonic cleaners, and stripping processes. Even the practice of bombing can create a violent chemical reaction capable of generating enough heat to damage some stones. In addition, heat reverses nearly every form of color treatment, and detergents and other solutions can remove oils, dyes, and similar enhancements. When working with any mounted stones, jewelers should remove the stones or use heat-shielding material. In some cases, it may be best to avoid heat-generating machines altogether.

The more common cause of breakage among gemstones is physical trauma, defined as an applied external force. Damage can range from the infamous paper mark on a stone (a minor surface abrasion caused by stones rubbing together in the stone paper) to complete shattering.

To avoid physical trauma, setters should consider a stone?s hardness and toughness. Hardness refers to a material?s ability to resist scratching or indenting and must be taken into account when a setter shapes and finishes metal around the stone. Will a file or abrasive wheel cause scratches? Gemstones that rank above 7 on the Mohs scale (e.g., alexandrite, topaz) usually won?t suffer scratches from steel tools, but they can be damaged with abrasive wheels. Some abrasives, like those found on cut-off discs, can damage stones with a Mohs ranking as high as 9, such as sapphire and ruby. As for toughness?the material?s ability to resist fracturing when physical stresses are applied?it becomes a consideration when tightening a stone or hammering a prong. Filing also can cause a problem. If the file teeth drag against a facet junction, the traction could cause a chip?even if the stone is harder than the file?s steel.

Next, setters must consider the force applied to the physical setting itself. While many types of settings exist?prong, channel, bezel, invisible?most share one common attribute: They bind the stone in place. How they accomplish this can literally make or break a job.

Putting a stone in a bind. The prong setting is the most prevalent setting in use today. To make it work, jewelers cut a seat in the prongs, then bend or move metal over the stone to hold it in place.

This is the point at which breakage most frequently occurs. As the prong tip is bent down, it makes contact with the stone and bends to match the surface contour. The inside surface of the prong distorts outward, and the outside surface elongates. Generally, the bend will occur at the seat?the point of least resistance?and as the inside surface metal in this area expands outward, it will ultimately make contact with the stone?s girdle.

When the point of contact is along the top or bottom edge of the girdle, it will put great physical stress on that location and increase the likelihood of the stone?s chipping. These chips may be better described as small surface splinters that extend from the girdle?s edge to either the pavilion or the crown. This is not a common problem, but it may show up if the stone has a particularly thin girdle or is a brittle species, such as emerald or tanzanite. When working with such stones, setters can avoid problems by using a round or bud-shaped burr to cut the seats, which prevents the girdle from directly contacting the metal around the seat.

The force applied against the stone increases throughout the bending process for each prong. As a prong bends, more of its metal comes to rest against the stone?s surface, and the point of least resistance shifts up the prong. The portion of the prong tip not in contact with the stone shortens. Since a short piece of metal is harder to bend than a long piece, more force needs to be applied, increasing the chance that a setter will use too much and break the gem.

All metals are ductile materials. They can be bent or shaped, and they have a ?memory? similar to that of a spring. If the amount of bending is below the material?s ?yield point??the degree of applied force that results in a permanent bend?the material ?rebounds? or returns to its original shape or position. To overcome rebound, stone setters commonly overtighten the prong tip to the stone.

Depending on the type of stone, overtightening can lead to breakage immediately, since it loads stress along the prong tip pinching the stone. Even if an immediate problem doesn?t occur, the residual stress in the prong tip often accelerates stress corrosion in the metal, which can lead to prong failure and loss of the stone.

Setters may try to compensate by pushing a prong tip straight down from above, after it has been bent into position and cut to length. If the metal is compressed in this way rather than elongated, the rebound direction changes as well. Instead of moving away from the stone?s surface, the prong tip maintains secure contact with the stone. Most often a beading tool is used in applying this force to get the prong tip tight to the stone.

But this practice, too, can cause trouble. If the required force for bending the metal and contouring it over the surface of the stone exceeds the gemstone?s ability to absorb the force, the stone will break. And since there?s no practical way to measure the amount of force being applied against the stone, the setter has to rely on feel when making the determination between not enough and too much. ?Feel? comes with experience. Setters who want to practice should consider using inexpensive foilback stones made of glass. They?re easily damaged or broken if too much stress is applied. Cubic zirconia can be used, but it tends to be more forgiving of rough treatment than the glass stones.

There are some preventive measures jewelers can take. Seat design can be changed to redirect applied forces in a way that makes these forces less likely to damage a stone. Jewelers should avoid a sharp ?V? notch (produced with a bearing or hart burr), which tends to focus stress along the girdle point. Instead, they can carve the seat with a setting burr, whose straight walls allow for a greater degree of lateral displacement over a longer area, reducing the amount of pressure exerted on the stone.

Some settings require different burr styles, such as a setting in which prongs are shared between two stones. The seats for these shared prongs often are cut with a 90° or 70° bearing burr, and a straight-walled setting burr is used for the outer prongs. With this arrangement, the setter can tuck the stone into the seats cut with the bearing burr, then pivot the stone down to rest flat. During the tightening process, the shared prong tips should not be adjusted much. The major tightening should occur with the outside prongs.

Heavy prongs create additional challenges. Generally, a stone setter will try to cut a seat that?s between 30% and 50% of the prong?s thickness. But in a mounting with prongs more than 1 mm thick, 30% is not enough. With seats cut to a depth of 50%, bending the metal over the stone requires less force, which reduces the chance of breaking the stone.

Finally, setters should make sure the prong tips rest flat against the stone along the entire length of contact. I have seen prong tips that, after bending, have bowed over the stone so only the very end makes contact. The most common consequence is a stone that loosens quickly. Stones that become loose in the mounting are more likely to sustain damage than those that are firmly secured, since a loose stone can have directional forces exerted against it that it isn?t designed to take.

Broken by design. It?s important to look at the shape of each stone to match its contours when cutting the seat. Some stones have a wavy girdle, while others may taper. In such instances, cutting the seat in a straight line?generally thought to be good practice?may actually result in making the stone more susceptible to damage.

Regardless of the setting or the cut of the stone, seats with uneven surfaces will not uniformly distribute pressure. Instead, that pressure will be localized at a single, focused point. Consequently, stones that traditionally can take a greater amount of force can be damaged.

When the seat is cut off-center, the prong will not fully support the stone. Instead, the part of the seat nearest to the center of the prong will be the major point of contact. This could cause the stone to loosen continuously during wear, no matter how many times the jeweler tightens it. In fact, continued tightening could fatigue the metal and result in a prong tip failure.

Not having seats on the same plane in each prong can result in unevenly distributed pressure. If one seat is significantly higher than the others, too much upward stress is put on the stone. Consequently, the adjacent prong tips pull down on the stone after tightening. And because of the misaligned seat, they won?t support the pavilion.

Damage doesn?t always happen during setting. During wear, prongs can be subjected to small bumps and vibrations. When one seat is higher or lower than the others, and the stone is secured from the top without an adequate foundation, slight deviations?even 0.1 mm?can create a focused stress point. If the stress point happens to align with a weak plane in the stone?s crystalline structure, the stone can be broken with relatively little external force.

Channel setting can create many of the same conditions, especially when baguettes are involved. The design of the baguette cut makes the stones susceptible to damage?their sharp corners can be chipped or broken off during setting. If the seat beneath the stone is not shaped to match the general shape of the stone?s pavilion, a stress point is created wherever there?s a high or low point in the seat. Since the corners of baguettes are thin and unsupported by the stone?s pavilion, the amount of applied force necessary to damage the stone can be exceeded?sometimes all it takes is the pressure exerted by a fingernail to push the stone into its seat. Seats must match the contour of the stone, so that all contact points bear equal pressure.

Bits of metal left in the corners of the seat?most notably in the beginning and end of the channel?also can lead to broken corners on baguettes. A small excess of metal, hardly noticeable when the stones are put into place, can cause a stone to break upon tightening.

Burrs also can prevent a stone from being completely secured. A small burr present in a channel setting, for instance, could keep the metal from being tightened completely; if that burr then becomes dislodged during polishing and closing, the stone will loosen.

Mounting troubles. Other problems can occur when the stone to be set is too large for the mounting. Let?s say you have two different-size baguettes, and a channel mounting that was designed to hold the shorter of the two stones. The larger baguette would require the removal of a great deal of metal. In such an instance, setters remember to modify the entire seat. Some may cut back only the top bar to fit the stone, ignoring the seat?s base. If they don?t extend that base to support the pavilion, too much stress will be placed on the stone, and breakage may occur.

Mountings designed with shared prongs also create problems. Setters must use different burr styles when carving the seats, and they must find a way to keep the stones from touching and damaging each other during the setting process. Just a minute space (as small as 0.1 mm) between stones is adequate to prevent chipping. And girdles should never overlap when they?re set at the same height. If they do, the stones will chip each other upon tightening.

The waterfall cluster, which uses a prong-and-wall combination to hold stones in place, also requires the use of different burrs. Seats are cut in the prong, and a shallow undercut is made in the wall. The most common tendency is to overcut the portion of the seat that goes into the wall of the mounting. Instead, prong seats should be cut with a setting burr, the wall seat to hold the girdle should be cut with a 45° hart burr, and the wall seat to hold the pavilion should be cut with a 90° bearing burr. In this way, the stone is held both above and below the girdle line, so it?s kept level and subjected to neither upward nor downward force?either of which could lead to breakage.

There?s no substitute for meticulous attention to detail. Whether cutting the seats, tightening a stone, or removing burrs, jewelers must remember the stresses a stone can undergo. If they do, they can then take the proper precautions to minimize breakage and make the job of setting a gemstone a much less stressful experience all around.

Gregg Todd is a master instructor for program development with Stuller Settings in Lafayette, La.

This article was adapted from a longer piece that appeared in the February 2000 issue of AJM magazine. Reprinted with permission.

See our new department, Bench Tricks, on p. 74 for more information on minimizing stone breakage.