Oval Center Stone

Practical Stone Setting, Part 20

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 publisher is responsible for injuries, losses, or damage that may result from the use or misuse of this information.
Alan Revere, director of the Revere
Academy of Jewelry Arts

1. This classic and basic mounting from Stuller is available in a range of styles for different stone counts and sizes. This typical model holds eight 2.5 mm side stones and one 4 mm–6 mm oval center stone in the 14k white gold mounting. A 14k yellow gold shank with a leaf pattern has been added. Inspect the settings to make sure the prongs are straight, spaced evenly, and the correct height. File the tops slightly to establish a uniform plane. The side stones are generally set first, as in this example, since access to them will be limited after the center stone is added. (See Practical Stone Setting, Part VII, “Small 3-Prong Side Settings,” JCK, April 2004, p. 117, for instructions on setting three-prong side stones.)

Fig 2. With a pair of dividers, mark the height of the seat on the inside of each prong.

2. Secure the work in an engraving block lined with wooden slats (paint sticks) for protection. Setters and bench jewelers use a wide range of devices to hold rings like this. A ring clamp held up against a bench pin is very popular. Rotating vises, dop sticks, ring mandrels, and other devices also are used for this task.

Inspect the oval center stone and note the girdle thickness: thin, medium, thick, or varied. This stone has a uniform medium girdle thickness. After checking the alignment of the eight prongs for the center stone, place the stone on top. It should cover the inner half of each prong when viewed from above. Since these prongs are short, there isn’t much room to accommodate different sizes, as would be the case with longer prongs, which can be pushed in or out. Remove the stone and set a pair of dividers at 0.8 mm (more or less). Mark each prong with a nick, indicating where the bottom of the girdle will rest when the stone is set. Ideally, after the seat is cut and prior to setting, the prongs should be approximately the same height as the table of the stone. (Later the prongs will be lowered.) Mark all prongs uniformly and clearly by resting one leg of the dividers on top and scratching a line across the inside of each prong.

Fig. 3. Use a setting bur to cut seats at the marks.

3. Look through your burs for the box marked “Oval Burs” and pick one the same size as the stone. Just kidding. That’s part of the challenge of this setting. Unlike round stones, which are easily set with round burs, we do not have any oval burs, or square or marquise-shaped burs, either. So the bur of second choice is round. Select one that will fit into the end of the setting so it touches all three prongs. In this case, a 3 mm bur just about rests in the end of the oval touching three prongs. The corner between the diagonal seat and the remaining vertical wall will be precisely at the mark made earlier. It’s more difficult to cut even seats across a number of prongs with a smaller bur. Dip the bur, as it’s spinning, into wax for lubrication. Hold the bur firmly and bring it to the setting while spinning just fast enough not to catch. Carefully bring it into contact. Pull the bur slowly sideways into the end of the setting, leaving a crisp faceted seat. Go one-third of the way at first, and then a little deeper—close to halfway, but never further. The remaining metal should be at least 50 percent of the thickness of each prong. Move the bur slightly to the left and right to cut the adjacent prongs. The key is to keep the bur very steady and make level cuts. In general, the bur is more controllable if it spins slowly, but fast enough not to catch.

This takes practice, so perfect your control on a piece of scrap before trying it on the real thing. Continue to cut seats for the remaining prongs and check for uniformity. Remove flashing on the sides of the prongs with a fine needle file or abrasive wheel in the flex shaft.

Fig. 4. Close the prongs with a flat pusher.

4. After checking the mounting again for uniformity and finish, removing all tool marks, and giving it a final polish, insert the stone. It should rest level with the height of the remaining prongs, level with the table. If the prongs are taller, that’s fine because it gives you extra leverage, but requires a little more cleanup. If the prongs are shorter than the table, finding the right spot to push and getting enough pressure could be an issue, but cleanup will be shorter. With the stone in place, use a flat pusher to close the prongs, one at a time. This short tool is made from 100 mm of 4 mm tool steel filed to a 1.5 mm–2.5 mm face. The face is textured by hammering it into a flat file prior to mounting it into a wooden handle.

Using your fingers for stability, rest your thumb between the vise, the work, and the pusher. Gently lean the pusher into one of the end prongs at about 45 degrees. Watch it to move slightly. Stop and work on the opposing prong, then another pair of opposing prongs. Tighten all the opposite pairs to the same (halfway) point, and then push them further. Always work opposites in stages. For the final round, raise the handle and push from a steeper angle, pressing the tips of the prongs onto the stone. Systematically proceed until all the prongs are tight and the stone is fully secure. Inspect your work with a loupe to make sure the tips of the prongs are in full contact with the stone. This is much easier when setting diamonds rather than softer and more fragile stones. Check the hardness of individual gem materials and take appropriate care before proceeding.

Fig. 5. Trim and refine the prongs with a fine-cut needle file.

5. If the prongs are long, they need to be trimmed. The height of prongs for a center stone this size should be slightly below the table of the stone and come less than 50 percent up the diagonal slope. Use a clipper or saw, if needed, to trim long prongs. Then use a cut No. 4 or cut No. 6 barrette needle file with safe edges to trim the tips and shape them. This needle file’s edges have been ground down on a flat abrasive surface (sharpening stone or flat sand­paper) and then polished so it will glide harmlessly along a stone’s surface while the adjacent surface cuts. Insert the file carefully, and then slowly file the tip of a prong with rounding, curving strokes (never flat and straight, which would leave facets). After partially rounding one tip, go to the next and bring it to the same point. Go around doing the same thing to all and then a second time, taking them all further. Remove all tool marks.

Fig. 6. Use a cup bur to finish the tips of the prongs.

6. Once the prongs are approximately round and uniform, select a cup bur to finish them. This type of bur has no teeth on the outside. Instead, it has teeth on the interior of a cup, which means that when it spins, it leaves a spherical ball on the end of a rod or prong. Cup burs come in different sizes, so select one that fits over the prong without much play. Lubrication helps, so dip the spinning bur into wax. Bring it lightly onto each prong, first from above and then with a wider circular movement that includes the very tip of the prong, where it meets the stone. (Note: Polish the ends of cup burs to a high luster before use by spinning into 600 grit and then polishing paper.)

Fig. 7. Completed setting.

7. Inspect the prongs with a loupe and make final adjustments as needed. Look specifically at the tips to make sure they are in full contact and uniformly round. Remove flashing with a graver or 6/0 saw blade. Polish minimally with rouge to preserve the crispness of the setting.

© 2010 Revere Academy of Jewelry Arts, Inc.
First publication rights assigned to
JCK magazine.