Well Cut Stones   |   Cutting Small Stones   |   Polishing the Back   |   Opal Carving    |   Treating Cracks
Well Cut Stones: In order to sell stones they must be well cut. What does this mean for opal?

Polish: Stones should be very shiny on the top, when finished, with no scratches visible under a 10x loupe.

Doming: The top of stones should be curved down evenly all around to meet the sides - no flat spots. If the top of a stone has to be flat (necessary if stone is directional or thin colour bar) then the top roll down to the sides should be minimal but even and smooth. There should be no sharp edge on the top or the bottom. The widest part of the stone, at its "girdle" or "waist" or "setting edge" should also be slightly rounded and the sides of the stone should not be dead straight.

Thickness:Stones should be of even thickness at both ends

Form and shape: If an oval is being cut, it should be smoothly rounded, even and symmetrical, with no straights on any side. It is common to use a templat to form up an oval, particularly if a calibrated size is being developed. However if this is not desired, or too much good opal would be lost, a more pleasing shape can often be obtain if the stone has very subtle shoulders at each end (ie slightly "compressed" at each end.

If the stone is a freeform, its form should have appeal in its own right - often achieved though slight rounding of longer, straighter sides. Cutting attractive free forms is a bit of a black art and cannot be easily taught or even explained.
Small Stones: Many cutters have difficulty with small stones. However, some professional cutters and traders make a good living out of them, selling the stones for earings and other small settings. Chinese cutters routinely turn out stones down to 2mm round - accurately calibrated by eye. Nothing is wasted! Good opal is expensive and hard to come by. Even the smallest stones should be cut!

The supply of smaller material is generally more plentiful and cheaper than larger pieces. As a result cut stones and set pieces are more affordable to the public, and can generate good profit margins.

Professional cutters routinely cut small stones, but have their own preferred way of mounting and cutting them. For dop sticks you will often see very small nails used, even match sticks, with epoxy glues. However some cutters like to use the same sized dop sticks that their hands are used to for bigger stones, but sand the ends of the dop sticks down to a small point (spinning them against the grinding wheel), then taper dopping wax down further to mount and fit the small stone. Magnification is invariably used.

To see how one small chip of good colour has been used as inlay with great effect click here

Tips and Tricks: The following tips and tricks have been provided to help cutters come to grips with small stones. These ideas have come from contributions to the "Lapidary Digest" (click here for more information) .
From Earl

"It takes a light touch during the roughing stages, and a well worn sanding belt before polish.

In my experience it is easier to get smooth cabs using worn silicon carbide 400 & 600 grit belts than it is using diamond. Of course there are hundreds of folks that will disagree, but that's one of the joys of this hobby: what works fine for me may cause a disaster in your workroom.

Earl English
From Kreigh Tomaszewski

"For smaller stones I find it easier to hold the stick very close to the stone, and by moving my thumb towards the tail of the stick (nail) I get the leverage to make a smooth rotation of the stone across the wheel. Moving the rotation point closer to the stone makes it easier to make the tight smooth curves needed for small stones. I just found the longer sticks made it harder because they hit my other fingers sooner when rotating.

I also agree with the suggestions about lightening up a little, starting on a finer wheel than normal, and using worn sanding disks.

"Beyond the issue of dop stick diameter, small cabs also need shorter dop sticks. I usually use a #6 or #8 finishing nail. Sand the head flat, blunt the point on the grinder, and use super-glue to affix the stone to the head. I set the stone upside down, put a drop of glue on the back, and hold it down with the nail until it sets. Some recommend a small scrap of newspaper between the nail and stone.

To remove the 'dop' stick, grasp the stone and dop stick between finger and thumb firmly, and rap the free end of the nail hard on a large metal object. I use the edge of my grinder. You want to be holding the stone and nail when done. I've only had a few chips where the glue pulled out a chip of stone,

Mark your slab with a template so you know the size cab you want and trim around it, but don't touch the line. Make sure your line was to size by using a pointed scriber (I use an sharpened length of aluminum TV ground wire).

Rough grind (or fine if softer rock) the edges left from trimming up to, but not quite touching, your scribed shape. You should have a square edge (90 degree angle) and a smooth outline shape that is just a hair too big. Use a fine wheel and carefully make a very small bevel (45 degree) around the bottom until the bottom is just a hair smaller than the template hole.

Now we start trimming the top edge to bring it to shape in steps. Imagine using a super sharp knife edge to trim off the rock from just above the bevel at the base to just inside the scribed line on the top. We want a hairline thin band between the top of the bottom bevel and the bottom of the top trim. Grind this around; it should still be just a hair larger than the template.

Repeat many times, each time lifting the base a little and moving towards the center at the same time with each 'cut' on your grinding wheel. If you do it right you will end up with a whole bunch of rings (when viewed looking down on the top of the cab), each the same shape (and centered in) as the template (and the stone is still just a hair too big to slip thru it). Now you have the shape roughed out and need to be off your coarse wheel.

So far we've been working around the stone. Now, on the fine wheel, we're going to work from the base to over the top. Each time we roll we start at a different point around the base (1/4 to 1/3 the way around), using a light touch so we trim the edges and not the small flat faces of the rings. Over and over, around and around. Each rotation across the wheel goes from the ring just above the base bevel smoothly until it is just over the middle of the top. Repeat until you have the shape smooth and the stone just barely be forced thru the template (but don't push it quite that hard) - the rest of the edge will disappear in sanding and polishing, and the stone will exactly fit your template.Do the same rotation sequence from base to top middle for each sanding grit. Polish likewise.

With small stones, especially round ones, and a little practice, you can align the dop stick roughly with the axis of the grinding wheel and 'roll it' to get the edge trims, tilting the base of the dop stick away from the wheel a little more with each level. It is a little harder with ovals to do this and keep the shape right.

If you do this right there is no way to produce a flat spot. If you do, you have not made a smooth motion through the entire length of whatever cut you were making or pressed too hard thru part of the stroke.

And if you want real fun, try cutting 2x3mm opal cabs for replacement side stones on rings. I'll give you a hint, a micrometer helps. The amount the first ring needs to be larger than the template (desired size) at the beginning can be calculated - its the amount that will be removed by the rotation grinding, sanding, and polishing... 80 grit is approximately 7.5 thousandths of an inch in diameter. 120 grit is approximately 5 thousandths of an inch in diameter and needs to remove between 20 and 30 thousandths of an inch of surface (you have two sides) to eliminate the scratches and pits of 80 grit. 220 grit is approximately 2.5 thousandths of an inch in diameter and needs to remove between 10 and 15 thousandths of an inch of surface (you have two sides) to eliminate the scratches and pits of 120

grit. 600 grit is approximately 1 thousandths of an inch in diameter and needs to remove between 3 and 5 thousandths of an inch of surface (you have two sides) to eliminate the scratches and pits of 220 grit. Aluminum Oxide 305 pre-polish is approximately .2 thousandths of an inch in diameter and needs to remove between .4 and .8 thousandths of an inch of surface (you have two sides) to eliminate the scratches and pits of 600 grit.

Polish is approximately .001 thousandths of an inch in diameter or smaller and needs to remove between .1 and .2 thousandths of an inch of surface (you have two sides) to eliminate the scratches and pits of 80 grit.

Kreigh Tomaszewski
Webpage at: http://Tomaszewski.net or http://Tomaszewski.net/Kreigh/Minerals/MineralLinks.shtml
From Craig Nielson

I would add that for very small objects, I start out on a 280 diamond wheel. The really coarse grits just seem to grind it down TOO fast, and give me flats, and other boo-boos before I know it!!! Be sure to use a soft hand in grinding and sanding, and check it often.

Another trick for small dops is to get some brass rod from your local welding shop to use as dops. They come in various diameters. By taking a piece of channel iron and cutting a slot in it at 90 deg. to the side just wide enough for a hacksaw blade you have a nice little miter box. By stacking in the rods you can cut a bunch at one time.

If you are cutting a bunch of small rounds that are of the size of the rods that you have, all the better. By using superglue and a piece of matchbook cover or postcard for a spacer just cut to the brass and you have your outer shape and size.

Craig Nielson
From Dick Friesen

This thread reminds me of the time a lady brought me a petit point ring with a stone missing for repair. I explained that I might have to use stabilized turquoise to match the color and that the time required to cut such a small stone was higher than you would expect from the size of the stone, but she was willing to pay the price I quoted so I took the job.

I thought matching the color was going to be my biggest problem so when the first piece of rough was a perfect match I should have suspected I was in trouble (Murphy's Law). It started with the trim saw; I had to hold the second piece with tweezers to keep it from dropping between the blade and the table, where the first one went.

The stone needed to be just a little under 1mm by 3mm and I could not figure any way short of carving a special piece of aluminum to get a dop stick for it and I thought making the dop stick would take longer than cutting the stone.

After several attempts to dop it I just grabbed my Foredom, a Cratex wheel, and my #5 Optivisor and carved it holding it in my fingers.

The lady got her ring and I got a LOT more respect for the artisans that cut those stones. If anyone knows how the Zunis cut those stones on the reservations I would like to hear about it.

Dick Friesen
From Hans Durstling

No-one has yet mentioned the fact that the wax itself can be shaped into a small dop. This is how I've seen it done in Idar. Simply gather up a gob of hot wax on the dop stick in the usual way, then with wet thumb and forefinger twirl the soft wax out into a cone shape. Let it harden enough to maintain its shape, touch it to the hot small stone, adjust.

Such a wax cone tip can be molded to any size, with cautious heating the stone can be leveled out and centered (can't do that with crazy glue!) and the stone doesn't need to be flat bottomed.

However to do small flat bottomed cabs in multiples and in predictable sizes I'll often use a number 4 flat head slotted brass wood screw held in a pin vise. Screw heads straight from the package may not be perfectly flat and may need to be touched up on a fine file. Using crazy glue, glue up quite a number at a time, and then insert one screw after another into the pin vise. It's quite quick. If you grind into the screw head the metal may burr up and pop the stone off, so the screw head should be lightly beveled. The slot in the screw head aids access of acetone to dissolve the crazy glue later.

Hans Durstling sinico@nbnet.nb.ca
(Hans Durstling does stone cutting -cab and facet- freelance writing and custom goldsmithing in Moncton New Brunswick Canada; he is an opal addict and this is a picture of "Millennium Bug no. 1," a recent piece with three opals in silver and 14kt gold. The two smaller opals, in the "bug" were dopped as described above".)
Polishing the Back: A question which often arises (particularly with fiddly small stones) is whether the back of a stone should be polished, for commercial reasons. The short answer is it depends.

It seems to be a habit of cutters on the South Australian opal fields, and Chinese factories, to polish the back of light opal stones. This does have value in giving the stones a clean "manufactured" or "finished" look.

Boulder opal is often polished on the back, for several reasons. Many people think the brown ironstone looks better polished and makes the stones look more attractive, thus making them easier to sell; also many boulder pieces are used in pendants where the back is visible (in fact, a polished or finely sanded well formed back would be advisable for most open backed pendant pieces of all opal types).

However professional cutters of dark base Lightning Ridge material in New South Wales often leave the backs unpolished, just well formed and sanded smooth, and volumes are sold this way to field buyers. Often the potch backs of stones from nobby origins can contain sand or pits which are best not highlighted by a high polish. In any any event, a fiery attractive stone will sell well regardless of whether the back is polished. There could be some marginal advantage in terms of "finish" for polishing the backs of lesser stones.

Polishing the backs of lighter base, crystal or transluscent stones actually diminishes the brightness and therefore value (as the stones are being sold loose) of these stones, by allowing more light in from behind. There is some advantage in leaving these unpolished on the back!

All that said, the polish should always extend from the top down to a point where any part of the stone in an open (eg claw) setting would be visible. Where a large amount of the stone is visible from behind a setting (eg pendant or brooch) there could be some value in polishing the back.
From Dan

Tools for Opal Carving: The best flexshaft tools I've ever found for sanding and polishing stages are the Nova Miniature Points from Diamond Pacific (800-253-2954). Basically these points are like Cratex, but with diamond grit instead of silicon carbide. They are BLOODY expensive, about USD $7.25 each. However, they last quite well, cut fast, and give excellent results. Their grit sizes are: 60, 140, 280, 600, 1,200, 3,000, 8,000, 14,000, and 50,000. The points come in flame and bullet shapes, both small and large. I buy the large flames and bullets in every mesh. It costs a fortune but it's worth it because of the time that you save.

For polishing larger areas of a larger carving (not opal), try the Diamond Pacific Nova wheels in the 2 3/8 inch diameter. They come with bushings that take the internal diameter down to 1/4 of an inch. Put a 1/4 20 bolt and a fender washer through the bushing - ta da! Mounts in your #44 handpiece. Again, these wheels are expensive, USD $50.00 each or set of 4 grits for $180.00, but they will polish slabs and large areas of a carving faster than you can say impending recession.

Hi-Tech diamond (805-522-6211) has a similar line of points, and they also have a line of mini-disks. Their process is to embed diamond grit in epoxy and spread it in a thin layer on a disk or felt bob. They are *much* cheaper than the Nova points. I keep a good supply of these on hand even though they are vastly inferior to the Nova points. The Hi-Tech diamond mini-disks are pretty good and inexpensive solution for polishing large areas of a carving.

Another product I like from Hi-Tech diamond is their pretty extensive line of diamond compound in a syringe. They take diamond grit and mix it with a toothpaste like material. You squirt very small amounts of it on a felt bob and start polishing. You cannot use this stuff with opal since it must be worked dry and will heat the opal too much. However, it is handy stuff to have around for other materials since there are many shapes of felt bobs/disks/pads/etc that you can put it on. I think I paid about $10.00 for a 5 gram tube at a recent show.
From Ivan

Treating Cracks in Opal: I have had some success with the method to be described. Not all opal is treatable.

I bought a small hand-operated pressure/vacuum pump from Cole-Parmer, Model 79301-10. It has a gauge which shows either vacuum or pressure. It also has a switch to change from vacuum to pressure. A semi-rigid hose was attached to the pump and to a heavy glass jar with a metal lid using things from the local hardware store. An airtight metal container will also work. The other thing needed is also from the hardware store. It is a clear glue curable with ultraviolet light. The glue was designed to provide invisible mends for glassware. Glass and opal have similar refractive indices. The one I have used is Duro Clear Glass Adhesive #CGA1 81190.

The process must be carried out in dim light. Opals which have been taken to prepolish stage and dried are coated with the glue and placed in the chamber. I draw about as much vacuum as the pump can provide and leave the vacuum on overnight. Then the vacuum is released slowly. Pressure is then applied. The pressure is held on for about twelve hours. Pressure is released from the chamber. The opals are cleaned of excess glue on the surface. Either isopropyl alcohol or acetone can be used. I prefer the alcohol because it seems less aggressive. The stones are then exposed to ultraviolet light. Here in Arizona that's easy to do with sunlight. After the glue is cured, the opals can then be given the final polish.

Ivan Saddler


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