Practical Stone Setting Part V: Beginning Prong Setting

Prong setting is one of the most common ways to set diamonds and other gems in jewelry. The concept is simple but the execution is not. The basic idea is to carve seats into prongs to hold a stone securely. Like most complex techniques, breaking the process down into steps makes it easier to learn the technique and to control your work.In the following elementary lesson on prong setting, master goldsmith and educator Alan Revere breaks down the procedures into the smallest increments to demonstrate the most elementary and important steps in prong setting. For this reason, instruction on prong setting includes several added steps to ensure accuracy, including the use of a file to cut seats in the prongs.

  1. This 14k gold cast mounting has one large four-prong setting for a diamond. The prongs are hefty and there are no other settings or obstacles to contend with.

  2. Using a slide caliper, the diamond measures 6.1 mm at the girdle, making it slightly under one carat in weight. This information will be important when the prongs are adjusted and the seat is cut.

  3. Standard procedure is to adjust the opening between the prongs so that the stone will fit properly when set. Ideally, a seat is carved halfway deep into each prong from the interior. The angled seat (or bearing) matches the angle of the stone’s pavilion, below the girdle. The remaining half of the prong becomes the vertical wall above the girdle, which is bent over the stone.
    Most prong settings can be adjusted to accommodate a range of stone sizes. Right now the opening is too large and the stone almost drops between the prongs, so it must be closed to fit. However, in order to accommodate a 7-mm stone or larger, this setting could be opened further. Or if closed, the prong setting could safely hold a 5.5-mm stone or perhaps smaller.
    The task now is to determine the correct position for the prongs to hold this stone. Use the caliper to measure the depth of a prong, from the exterior to the interior. In this setting the prongs are 1.2 mm deep and .9 mm wide. Here is how to calculate the correct position for the prongs mathematically. In this case, half of each prong will become the vertical wall, outside of the stone’s diameter. If the prongs are 1.2 mm deep and half of each becomes the vertical wall, then add .6 mm + .6 mm (for the two vertical walls) + 6.1 mm (for the stone) = 7.3 mm for the outside of the prongs. This is the correct measurement for the outside of the prongs.
    Since the prongs are now further apart, use a pair of chain nose pliers to squeeze opposing pairs of prongs inward toward each other. Check to ensure that the outside of both pairs of opposing prongs measures 7.3 mm. Check and correct for symmetry and alignment as well.

  4. Use a flat file to level off the tops of the prongs, bringing them all into one flat plane.

  5. Check the fit by placing the stone onto the setting. At this point, because the top portion of the prongs is straight and vertical, the stone should sit covering nearly half of the prongs. If the prongs were angled outward, which is very common, the stone would cover less of each prong, and perhaps even just barely rest corner-to-corner on top of the prongs. This is to allow for the stone to drop into the right position in the angled prong at the point where it will be held. In all cases, an imaginary line dropped straight down from the edge of the stone should cover half of the prong at the seat.

  6. Because this exercise is a demonstration of the basics, some steps that are not followed by experienced setters are included. In this case, for example, the heights of the seats must be indicated for cutting. Generally an experienced setter just uses the “eyeball” method to determine where to make the seat, without taking the time and effort to lay out a guideline. But it is easier in the long run to inscribe lines at the same height on all the prongs.
    Set a pair of dividers at about 0.9 mm. Place one leg on the top and drag the other leg very lightly across both sides of each prong.

  7. Another added step will make things easier. Use a saw to begin each seat from the inside of each prong, even with the lines. Check for accuracy and go only one-quarter of the way into each prong (about .3 mm deep). It is very important that these cuts be level with each other.

  8. At this point, most working setters would grab a bur and use a flex shaft to cut seats into the prongs. But we are going to slow down the process further—and ensure accuracy—by using a #4 cut square needle file to create the seats. This will afford greater precision. In order to do this, one must understand that the angle at the girdle of a brilliant-cut stone like this diamond is just about 89 degrees. Therefore, a square needle file with 90ºcorners is nearly perfect for the job.
    Place the tip of the file in the saw cut, rotating it so that the four corners are pointed exactly up, down, left, and right. Any variation in this orientation will leave seats that do not match the shape of the stone. With gentle pressure against the prong, begin to file a notch in the saw cuts. Do not let the file touch the other prongs as it will damage them. File until the cut is about 40%-50% into the prong. The seat should actually cover less than half of the prong—leaving at least 50% for the wall. Use a caliper to verify that the distance between the interior corners of the opposing seats is 6.1 mm. The seats should all line up in one plane as the ring is viewed from the side and rotated.

  9. Carve away the top of the seat so that the stone can drop in place. Do this with a barrette needle file held in a near-vertical position. While carefully avoiding the bottom of the seat, hold the file nearly vertical so that you can carve the top part back. To limit abrasion to just the vertical wall, polish the edges of the file. It is not important to remove all of the metal so that the wall is truly vertical, because the stone can be slipped into place with the walls angled inward slightly. This is preferred, because the stone cannot move as much during setting and the metal does not need to be pushed as far.

  10. At this point the stone should slip tightly into place with a little pressure. Check the fit and adjust as needed. Prior to actually inserting the stone for the final time, remove the flashing (the rags of metal that remain from filing). Also, polish the interior and sides of the prongs as well as all other areas that might be more difficult to reach after the stone is set. Insert the stone and press it downward to sit squarely on all four prongs. For a large stone like this, rotate it while viewing from above, until the prongs are all sitting at the same point of corresponding facets. Adjust as needed.

  11. There are numerous ways to close the prongs. In this project, we’ll begin with a pair of chain nose pliers in a two-step process. To help prevent slipping, sand the interior of each jaw with 240-grit paper, leaving a fine texture to grip the metal. Coming from directly above, place the pliers over the setting and outside two opposing prongs. When you squeeze, the angle of the jaws forces the tips of the prongs inward over the stone.

  12. Now rock the pliers back so that one leg lies along the outside of a prong below the girdle and the other leg meets the opposing prong above the girdle. Squeeze and watch the metal move in. Rock the pliers back and repeat for the opposing prong. This should bring the material in close, but probably not at the tips of the prongs.

  13. The tips of the prongs have to be closed in, but the pliers do not have the leverage to force the metal in the correct direction. Use a prong pusher with a coarse face (to prevent slipping) to finish the job. Place the working face on the tip and force the metal downward onto the stone.

  14. Check and adjust for contact, symmetry, and alignment of the hefty prongs. Use a flat needle file to trim the excess metal so that all four prongs are at the same level, about 80% of the height of the crown, the vertical distance between the girdle and the table.

  15. Use a #4 or #6 needle barrette needle file to trim the fronts of the prongs where the tips cover the gem. File each tip back so that the prong covers less than half of the slope above the girdle.

  16. Continue to carve the top of each prong so that when viewed from the side, each has a smooth, even, curved contour that blends into the straight outside of each prong below the girdle. In this case, leave the sides of the prongs flat. The tip of each prong should look like the side of a small cylinder, rather than a ball, as most prongs are commonly finished.

  17. Use rubberized abrasive wheels to correct and clean up your work. Follow by buffing with tripoli and then polishing with rouge. The finished prongs should hug the stone for security, with no spaces or gaps between the metal and the gem. The prongs hold the stone securely while showing it off to maximum advantage.

© 2003 Alan Revere First publication rights assigned to JCK magazine.

The procedures in this article are standard practices for bench jewelers at this time. If not executed properly, however, they can cause harm. Neither the author nor the publisher is responsible for injuries, losses, or damage that may result from the use or misuse of this information.