Cat's-eye Tourmaline
In this case, beauty is in the eye of the beheld.
by Hans Durstling

Among the stones that can be cut to show a star or eye effect, most everyone is familiar with star sapphires, Idaho star garnets, and of course the ubiquitous tiger-eye. Even opals can sometimes cat’s-eye. But it was not until I visited my friend Carol Bova that I learned tourmaline could also yield a very attractive cat’s-eye stone.
By now, I’ve cut quite a few of these — both on Carol’s Genie and on my own Graves CabMate. While it isn’t rocket science, there are a few quirks to watch out for.

Photo of finished piece by Hap Sakwa; all other photos courtesy of the author.

Preparing the rough.
Star and cat’s-eye effects are caused by microscopic, directionally aligned inclusions within the crystal. So, you need to orient the rough to take advantage of the alignment. In corundum (ruby and sapphire), the inclusions are microscopic needles of rutile that run perpendicular to the long axis of the crystal and intersect at 60° to give a six-rayed star. Hence the corundum rough, which is typically barrel-shaped, is sliced at right angles to its length to get a well-centered star. In tourmaline, the inclusions are oriented exactly the other way — they run parallel to the length of the crystal. So, to get the cat’s-eye effect, the typically rod-shaped rough crystal needs to be sliced along its length.

Some tourmaline crystals are more fibrous than others. Cover the rough with water, in a translucent plastic container, and shine a light on it from below. Select those pieces that show a uniform, fine, silky sheen — they will cut the best stones. Others will have larger, open channels or tubes running through them that are unsightly on the surface of a finished gem and collect polishing compound. Such open channel parts, along with other defects, should be trimmed away.

Trim away the tip of the crystal also, which often is so fibrous as to be downright “brushy.” Nor are the silky, “good” fibers uniformly distributed within the crystal but often concentrate in zones that end abruptly on an oblique plane, parallel to a termination face. With the half crystal wet, these factors can be judged, and you can see where the cat’s-eye line is going to be. This should be centered on the dop, so that the line is centered also in the finished stone.

Dopping needs to be level.
To cut an oval with the cat’s-eye running the length of it requires a fairly “fat” piece of rough. Therefore, as a rule, your finished stones will be round. Since a tilted base will give you a stone that leans to one side and has an off-center eye, you need to dop the rough so that it sits level and perpendicular to the dop stick.

This is a surprisingly fiddly process, particularly for small stones. It is not immediately evident where the green stone ends and the green dop wax begins. Plus, the wax runs up onto the sides of the stone, concealing the base.

To deal with this, I use two techniques. For the first method, place the sliced rough warming up on the dop-stove bottom upward. Take a small dop stick (a piece of 1û8" dowel works well), gather a gob of wax on it, and with wet fingers, twirl the wax out into a slender cone.

When the wax cone has hardened, touch it to the base of the hot rough. The rough is hot enough to melt the wax at the point of contact without flowing it all over the stone. This way you can see where the base is and level the stone while the wax is still pliable.

The second method is to stick the rough onto something you know to be flat. This is particularly effective for very small stones that are next to impossible to level by eye on dop wax. Using small, flathead machine screws, glue the stone to the screw head with cyanoacrylate instant glue. Hold the screw in a pin vise.

Under water, the cyanoacrylate bond will weaken in time, but the stone is rarely wet long enough for this to be a problem. If it is, however, use a little Vaseline® to waterproof the joint.

Keep it cool and watch for dimples.
Rough and fine grinding present no particular challenges, except perhaps getting the stone accurately round. Since the stones are small, start on a fine wheel, so as not to remove too much material in each pass. Follow with two courses of sanding on a flexible disc or wheel, first at 600-grit, then at 1,200.

This is where a peculiarity becomes apparent — the tourmaline is particularly prone to what I call dimpling. No matter how carefully and uniformly you’ve sanded at the 600 stage, the prepolish gloss of the subsequent 1,200-grit sanding will still reveal small, shallow depressions which take a while to sand out. Here, the moral is sand-and-look, sand-and-look.

If you’re using a Genie, take great care that the 1,200-grit resin bonded diamond wheel stays wet, wet, wet. The frictional heat on that wheel often results in drying at the point of contact. And the moment this happens, the stone will drag in the resin and will instantly crack. I’ve lost quite a few stones that way and it gets quite frustrating. This however does not happen on my Graves CabMate when using a 1,200-grit flexible diamond disc at the slower speed setting.

Wipe the stone dry on your forearm and check it under a bright light. If no dimples remain, you are ready to polish. If dimples are visible, go back to 1,200 and sand them out.

Polish and finish.
Any polishing method will work — with the caveat that the almost inevitable, minute, open channels on the surface of the stone may collect powder-type polishing compounds, like aluminum or cerium oxide. This discoloration is then virtually impossible to get rid of. What works best for me is to polish on a 3" diameter hard felt wheel with grease-based diamond compound, either 14,000- or 50,000-mesh. Water is not used and the felt builds up heat quickly, so a light touch is in order. An overnight soaking in acetone will detach stones that have been glued with cyanocrylate.

The final step is to cut a small setter’s bevel around the base to get rid of the sharp edge. Here, again, the “twirled-cone” dop wax trick is helpful. Heat the stone (base down) on the dop stove and simply touch it with the tip of the dop wax cone. The stone will stick very quickly and that leaves the base well exposed for light beveling on the fine grinding wheel. The base itself is left unpolished; the cat’s-eye works better that way. The result is an attractive and uncommon cat’s-eye stone.

________________________________________________________________

Feature Stories & Artist Profiles | Step-by-Step Projects & Workshops | Technique Articles | Business Articles | New Products | News, Conferences, & Deadlines | Take our Reader Survey

Articles & Workshops | Bead Fest | Jewelry Classes | Show Calendar | Search for Products & Suppliers | Classified Ad Deals | Advertising Rates & Information | Media Reviews | Store - Books & Back Issues | Contact Us | Links | Site Map