This cast ring from Stuller features an interesting design in which the shank flows directly into the six prongs of the setting. The prongs are square in cross-section, and slightly thicker than normal. Unlike most prong settings, which are conical or flared at the top, the prongs in this design run straight up and down. This design makes it an excellent project for learning the basics of using a bur to carve the seat for the stone.

Unlike bezels and channel settings, most prong settings can be modified to accommodate a range of sizes. Merely moving the prongs in or out symmetrically is all that’s required to make small changes in the size of most prong settings. As they are moved, the angle of the prongs will change. What is most important is that the prongs are situated so that at the point where the stone sits, it covers half the prong.

1. This ring is intended to hold a 4.5-mm round stone. But with a little adjusting, it could easily be opened or closed to hold a small range of sizes. By pushing the prongs inward or outward, the setting could comfortably accommodate stones from about 4.2 mm to 4.8 mm, or more.

2. The key is that the girdle of the stone covers half of each prong at the seat. So in general, the prongs are moved in or out, so that when viewed from the side, an imaginary line dropped down will indicate the size of the seat required to hold the stone.

3. At the point where this imaginary line reaches the seat, it should be covering no more than half of the prong. Begin by adjusting the prongs with a prong pusher or a pair of pliers. You can move them in or out, and also correct any misalignment.

4. File the tops of the prongs into a flat plane. Examine the fit of the stone over the prongs from above. Since the prongs are vertical, the view from above will be the same as the view lower down at the seat. Fine-tune the prongs so that they are symmetrical and the correct size.

5. Because the prongs are somewhat thick and the metal will resist being closed in over the stone, it is a good idea to set the stone a bit lower, thereby leaving the prongs longer than what is minimally needed. The added length, which will be trimmed later, adds leverage when tightening the prongs.
   Now set a pair of dividers to the height of the seat. (Note: Most professional setters skip this step, but for learning, it is an important guideline for added accuracy.) Hold the divider up to a slide caliper and set the tips to a distance of 0.8-0.9 mm. Transfer this measurement to the sides of the prongs to help you guide the bur to the desired height. Make the marks as light as possible on both sides of each prong. This is the point where the seat will be carved and the girdle of the stone will rest.

6. A “setting” bur is used to cut a seat with side walls that are vertical. It is in the shape of a cylinder with a cone on the end. The angle of the cone is nearly the same angle as diamonds. And so, when you push this bur straight down into a setting, it carves a perfect round seat with vertical walls. Setting burs are used in prong settings and tube settings. Select a bur that is the same size or slightly smaller than the stone, but not larger. Since burs rarely match the size of stones exactly, always drop down, rather than going up in size. In this case, a 4.3-mm bur was the closest available.
   Secure the ring in a ring clamp or other holding device in preparation for burring. Put the bur in the flex shaft and lubricate it with wax. Without spinning, hold the bur directly above the center of the setting to make sure it is the right size. Now turn on the power by stepping on the foot pedal at a slow speed. The speed should be just fast enough to prevent the bur from grabbing. Bring the spinning bur into contact very slowly, testing the cut to make sure it is centered and leaving the same mark on all the prongs. Adjust and continue to bur very slowly—these burs can rip up metal and lower the seat to the top of your bench pretty quickly if you aren’t careful. With great attention to what you are doing, lower the bur to the marks, and check it from all angles.

7. If the bur is exactly the right size, the stone will drop into place with no pressure. When using a bur that is slightly smaller, it is common to have to open up the side walls by moving the bur systematically and gently into each prong a bit further. Prior to inserting the stone, remove any rags of metal left on the sides of the prongs during burring of the seat, as well as the lightly scribed lines.

8. These prongs are thick and resistant to movement. Begin to close them in with a pair of chain nose pliers. Coming down from above, place the jaws on the outside of opposing prongs. Now rock the pliers toward one of the prongs, letting the jaw rest along its length. With the other jaw coming in at a steeper angle on the top of the opposing prong, apply pressure, closing the prong over the stone. Try to fold the entire top of each prong down tightly over the stone. Now rock back and press the opposite prong in. Continue to work opposing pairs until all of the prongs are closed in and over the stone.

9. Depending on the alloy and the way the setting was made, the metal will recoil and spring back to varying degrees. This is called “metal memory.” 14k has a greater memory than 18k; white gold more than yellow; die-struck more than cast settings. Greater memory means that the metal is more resistant to deformation and will tend to bounce back after being formed. Metal with more memory requires more force to move it permanently.
   Often, pushing prongs inward does not exert the required force or leverage to close the tips. For this reason, we have left the prongs on this ring taller than usual, to offer greater leverage for setting. The result is a tighter stone, and a little bit more filing.
   It may be difficult to completely close the prong tips with the pliers alone, so a prong pusher with a flat face and a short handle is the next step. The face of the pusher that comes in contact with the prong is left rough with file marks and not polished, which would make it slippery. With your hand wrapped around the tool and both hands stabilized on the ring clamp and the work, bring the face of the pusher down from a steep angle (about 75°-80°) and press downward. The added leverage helps you close over the tips.

10. Once the metal is down completely, it should be trimmed to the proper size and shape. Use a #4 cut barrette needle file with polished safe edges to trim all the prongs down around the stone. The finished prongs should be about 75% of the height of the crown (the distance between the girdle and the table). Systematically file the prongs: First, file them back from the center of the stone, then file them all down a bit so that they are even in height and size. Now use the same needle file to round all of the prongs so that the tips match and look hemispherical.

11. Go to a finer #6 cut barrette needle file with safe edges to fine-tune the shape and give the prongs a finer surface in preparation for buffing. The final rouge polish leaves the metal shiny and reveals a perfectly set diamond.

© 2004 Revere Academy of Jewelry Arts Inc.

First publication rights assigned to JCK magazine.