We’ve discussed in other articles how precision cutting can make a big difference in the optical performance of a gemstone when cut from the rough, and how recutting diamonds and colored stones can be a useful tool to repair damage. This time, we will demonstrate how a skilled lapidary with proper equipment can take an average or even below average native-cut, commercially cut, or otherwise poorly cut gemstone and make it into something truly special and compelling.
To demonstrate, longtime IGS Gold Member Daniel Stair will be recutting a commercially cut mixed oval spinel that currently weighs 5.24 carats. Here’s the deal. The standard mixed cut is used for most all gem shapes and consists of a bottom (pavilion) that is step cut and a top (crown) that is brilliant cut. Step cutting is done using rectangular or square shaped facets. Brilliant cutting is done using triangular, diamond and/or kite shaped facets. This spinel below is a prime example of some great gem material poorly cut.
The stone will be recut using a Facetron® faceting machine shown below.
This is probably the most popular faceting machine for custom gem cutting. On the upper left is a water tank that drips water coolant onto the spinning lap (cutting or polishing disc) below. This flushes away debris during cutting and helps to keep the gemstone cool. Below is a spinning lap (polishing lap shown here) that sits inside a silicone dish designed to stop water and debris from flying all over the cutter, wall, etc. The dish has a drain hose that is connected to a container that catches all the water, etc. On the right is a mast that holds the faceting machine head. The head is used to control angle, rotation and depth of each facet as it’s cut. Angle is adjusted on the right using the silver hand crank. Angle is displayed in the little window next to the crank. The rotation of the arm is adjusted using an index gear that is divisible by the number of sides of the finished stone. The gear can be seen at the top end of the arm, just below the dial gauge used to control the depth of each facet. Toward the front (on the right) you can see the on/off switch and the dial used to adjust the rotation speed of the lap.
To prepare the stone for cutting, it is wiped off with an alcohol swab to remove all traces of oil that might be present.
Next, the stone will be stuck to a flat ended dop stick that is keyed to fit into the end of the faceting machine’s arm where it will be held in place during cutting and polishing. This is done by heating up the brass dop using an alcohol burner, wiping off the old wax, then putting the dop into a block of new wax and getting a nice gob of melted wax stuck to the end of the hot dop stick.
A transfer jig with a cone dop is then used to help line up the stone on the dop stick to make sure it is nicely centered. This reduces the loss of face up size and carat weight to a minimal amount during recutting and may be the most important step in the whole process. It can take a number of tries to get the stone perfectly centered.
Next, the dop holding the stone is plugged into the end of the the faceting machine arm and the key shown in the lower front of the photo is used to lock the dop in place. Now, we’re ready to start cutting (grinding flat spots actually) using a diamond impregnated disc or lap.
The first thing to do with this stone is establish the shape by cutting the edges, also known as the girdle. In this case, we’ll be converting the oval to a more brilliant, modern custom cushion emerald design.
It’s kind of hard to see, but the stone now has flat edges where the curved girdle used to be. In the photo, they appear a bit lighter in color than the polished surfaces.
Once the girdle and shape are established, it’s time to cut the bottom of the stone. Often, a diagram is used, but in this case, it’s all being done without instructions to get the new design to fit the shape of the existing stone. First, the facets are cut around the stone to establish a point on the bottom.
Next, break facets are cut around the bottom edge of the girdle so the stone will sit flat in a setting. I’m also adding 8 little facets to the point on the bottom to increase the sparkle of the finished gem.
The last part of cutting the bottom is polishing. In this case, it gets a rough polish using 8K diamond followed by a final polishing using 100K super fine diamond powder and a bit of oil.
Here, you can see the bottom is partly polished and that helps to reveal the design a bit.
Once the bottom of the stone is completely polished, the dop holding it is removed from the arm of the faceting machine and placed in the fixed end of the transfer jig. Note that the bracket on the left is not fixed and slides on the two black rails.
We then repeat the process of getting wax on the end of a dop using the alcohol burner. In this case, we’re using a dop that has a cone shaped receptacle in the end. Once the wax is on the end, it’s put into the sliding end of the transfer jig, pressed up against the newly cut pavilion of our stone, and an impression is made in the wax. After that, a tiny drop of super glue is put in the wax impression to make sure the dop sticks to the stone well.
The receiving dop is then slid into place against the stone and it’s time to take a 10-minute break while the super glue has a chance to dry. After that, the dop on the right will be held over the flame to heat it up just enough to carefully soften the wax so it can be removed, leaving us with a dop holding the stone ready to have the crown cut. This completes the transfer.
That dop is then plugged into the end of the faceting machine arm just like the last one, and we begin cutting a design for the top, or crown, of the stone.
Once that crown design is cut, it is then polished using the same process as the bottom of the stone. The difference here is that we will next need to cut a new table or flat spot on the top of the stone.
To cut the table, the stone is removed from the arm of the machine, and is replaced in the arm by a special 45 or 90 degree dop holder, which is the big metal block shown above. The dop is then plugged into the bottom of that allowing the table to be cut on the top of the stone.
After the table is cut and polished, the dop is removed and the recut of the spinel is complete. All we need to do now is heat up the dop to soften the wax, remove the stone and clean the dop wax and super glue off the bottom. Sadly, the dop wax makes it look black through most of the cutting process, so one really doesn’t know what it will look like until the reveal at the very end.
And there we have it. The spinel is now much more beautiful in person, and has gone from 5.24 carats to 4.09 carats. 1/3 of the weight, but very little of the face-up size is usually lost when doing a total recut to a similar shape. Most of the weight loss is from removing the “belly” on the bottom of the stone that commercial cutters use to keep it heavy, since stones are sold by carat weight. This stone is almost completely eye clean in person. The inclusion you see on the side of the stone is actually a tiny Zircon crystal inside of the spinel!
Before and After (Still)
Before and After (Video)