American Opal Society

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Opal Cutting

Article by Clifford Coan

Intro

Opal has long been regarded as the fifth precious gemstone. (The other four are diamonds, emeralds, sapphires and ruby). It has also been regarded as the most beautiful and exciting of all gemstones by many. Reasons for this is the mysterious change of colors as the stone is moved around and the brilliance of the different colors it contains.

How the Name "Opal" Came About

The word "opal" is an ideal starting point for this talk: Most mineralogical texts say that it derived from the sanskrit "upala" meaning a precious stone. A tamed rendition, actually.

Opal was introduced into Greece during the first century B.C. (The Greeks and Romans probably obtained their opal from Turkey or India). The Greeks called it "opallios" a word coined from two other words, the first of which gives us words such as opaque and optical, while the other gives us words like alias and alter. Thus "opal" literally means 'to see a change' (of color).

The Indonesian name for opal is "kalimaya", derived from the Sanskrit 'maya', an illusion, and the Javanese 'kali' which moans a river. The result is a "river of illusion". The Aztec term ton Mexican opal means "the hummingbird stone", for its change of colors was likened to the sheen found on the plumage of that bird.

What Opal Is

Opal is a true precious stone which occurs in many varied forms. Opal is amorphous silica with a water content varying from one to twenty percent, depending on the porosity and degree of hydration. Precious opal usually contains from six to ten percent water (A more complete description will follow later).

Opal may be dull and valueless, in which case it is called common opal. Common opal occurs in abundance throughout the world. One form of common opal found in association with opal of value, is "potch". In color, potch may be white, gray, black, amber; it may be honey-colored, watery clear, or a mixture of these colors.

About 95 percent of opal from the opal fields is potch. Only 5 percent is of any value. Of this 5 percent about 95 percent is mediocre grade, with only 5 percent of real value. It is this small percentage- 5 percent of 5 percent that constitutes the magnificent opal which we call "precious opal". (No wonder precious opal commands such a high dollar price). Any other opal showing a play of color, but which is not equal in grade (color in intensity and purity) to the tops, is "noble opal".

Summary

Opal is a precious stone.

Opal Types

Top grade solid opal is PRECIOUS OPAL. Other brightly colored opal is NOBLE OPAL.

The precious-opal classification refers only to rough opal from which SOLID stones can be cut, or to finished gems which are solid stones. Top-grade doublets or triplets are classified as noble opal, not precious opal, because their opal content is backed with other material.

Solid opal is opal, which is of natural occurrence and solid enough to be set as a gem. Any pieces which need reinforcing (as happens in a doublet) are not solid opal, nor are any pieces which have a non-opal backing, including boulder opal with a backing of ironstone. An opal backed with opal is considered a solid, not a doublet.

There are other forms which cannot be classified as solid (thus not precious opal). These include any materials which show small detracting patches in the face of the cut specimen and which are either non-opal or lacking color play.

Where Nearly All Our Precious and Noble Opals Come From

Both precious and noble opals are mined in several areas in the world. A few of these areas are worth mentioning here.

Virgin Valley (Nevada) opal: This opal has long been acknowledged as some of the most beautiful opal in the world. However, the amount of opal produced for jewelry from this area is very limited. One reason for this is that most Virgin Valley opal cannot be cut and placed in jewelry because it either fractures or crazes when placed against a cutting wheel. By using a long term stabilizing process some of this opal can successfully be cut into finished stones and these stones are usually classified as precious opal. Since only a small percentage of this opal can be stabilized for cutting most of the opal mined in Virgin Valley (Nevada) is kept and displayed as beautiful specimens only.

Spencer opal: This opal is mined in Spencer, Idaho. This opal is formed in very thin layers, usually a millimeter or less. Consequently, most of it is used to make doublets or triplets. On rare occasion some is mined that is thick enough to cut a solid stone. Most of the "solids" would be classified as precious opal due to their intensity of color and beauty. The doublets and triplets are usually very brilliant and beautiful but since they are not solids they would be classified as noble opal.

Honduras opal: At one time this opal was in demand but at the present time the opal mined in this area has to be treated to enhance its' appearance. For this reason it has lost favor with most knowledgeable buyers.

Since the late 1800's Australia has boon the main source of opal (both precious and noble) for opal lovers the world over. In July of 1889 Australian T.C. (Tully) Wollaston took 60 brilliant pieces of Australian opal rough to London, England and succeeded in selling them to an international jewelry firm to be cut for jewelry. That sale was the beginning of the recognition of Australia as the world's source of opal.

There are several reasons for Australian opal being the world's source of supply. A few important reasons are: 1. Its beauty is known the world over. 2. Nearly all of it is cuttable for use in jewelry; 3. The supply is not limited. New fields are discovered frequently to augment the availability of Australian opal; 4. Mining techniques are being improved constantly to insure maximum recovery, both in new fields and some previously mined areas.

Destroying Some Myths and Complaints Frequently Heard About Opals

  1. Opals fracture easily. This is not the opal's fault. When it does occur it is usually because the opal is not cut properly or the mounting it is placed in is improperly manufactured. Opal is about the same hardness (5 to 6.5 on the hardness scale) an amethyst and jade yet you seldom hear of them fracturing. Chief reasons are due to the way they are cut (faceted) and the way they are mounted.
  2. Opals are unlucky. In 1829 Sir Walter Scott published a novel, "Anne of Geierstein" in which he used an opal to reflect the changing fortunes of the heroine. Critics of his novel conjured up the idea that opal could have properties of evil influence due to some of the heroine's misfortunes. This not only cut the value of opal in half at the time but amazingly this myth started by book critics in 1829 still exists today.

Some of the Do's and Don'ts Regarding Opal Jewelry

  1. Be suspicious of any opal jewelry that has the back covered with gold. Chances are this gold backing is done by some unscrupulous seller to hide the fact the opal is a doublet. Doublets are many times less valuable than solids. Be able to see the stone from both front AND back.
  2. If you store your opal jewelry in a safe deposit box put it in a small jar of water or put it in a zip lock bag with a piece of wet cloth enclosed. A safe deposit box is one of driest places there is and gemstones have been known to craze there.
  3. Do not wear opal rings when using liquid detergents. Opal is porous and the liquid detergent will penetrate the stone causing dullness.
  4. Do not soak opal in oils, such as glycerin. Liquids that are oil-based may cause moisture in the stone to be drawn out.
  5. After purchasing opal jewelry get an appraisal by a qualified gemologist. This will provide you with the information for insurance AND let you know if you made a good purchase
  6. Some jewelers tell you to soak your opal jewelry in water frequently. It won't hurt the stone but I feel it is completely unnecessary. First of all when an opal goes through the various polishing (usually diamond polishing) stages the surface of the stone is fused therefore water stands little chance of penetrating the stone. I have opals that I cut and polished eighteen years ago that have not since boon in water and show no ill effects.
  7. Due to handling over the years an opal (like any precious stone) may lose some of it polish. More often than not the top of the opal can be re-polished in the mounting. Check with a qualified lapidary (who is familiar with opal cutting) before removing or replacing the stone.

If the opal is cut properly and placed in a properly manufactured mounting there is no reason why it should not give you a source of enjoyment a lifetime provided it is given the same care as any other gemstone.

What to Look for When Buying Opals or Opal Jewelry

If purchasing opal jewelry

  1. If purchasing cut opal gemstones for investment, purchase only the top grade (that opal classified as precious opal). There will always be a market for properly cut precious opal and the appreciation has been excellent.
  2. Learn the proper way an opal should be cut. If cut improperly, a knowledgeable buyer will either not buy it or offer much less than it would have been worth if cut properly.

If purchasing opal jewelry

  1. By looking at backside of opal in mounting make certain opal is flat against seat of finding all the way around. If it isn't flat it will fracture in the future.
  2. Check opal at each place prongs contact opal for fractures. Inspect prongs to make certain they are flat where they contact stone.
  3. Never purchase jewelry when back of stone s covered with metal (gold). You may be looking at a doublet and unless you are made aware that it is a doublet and priced accordingly you could pay far too much for the piece of jewelry. This is done frequently by some unscrupulous or unknowledgeable sellers.
  4. If bezel and prong mounted make certain bezel covers lower third of stone. Again, make certain prongs are flat on side that contacts stone.
  5. If bezel mounted, check accuracy of bezel (on top) completely around stone. If there is a gap on one side of stone it may come loose when epoxy ages, and stone could fall out.
  6. Opal rings should have the stone recessed to protect from being bumped or hanged against other objects.
  7. Stones in pendants should have protection around the edges. Back should be partially covered. Recessing not necessary.
  8. Opals should be mounted in gold.

What Cause the Brilliant Color in Opal?

In the mid 1960's a group of scientists at the Commonwealth Scientific and Industrial Research Organization (Australia) solved this mystery and put an end to all previous theories.

By using a newly developed electron microscope which magnified 30,000 times they found opal consisted of tiny spheres of silica ranging in size from 0.00005 mm to 0.00004 mm. By comparing common opal (colorless potch) to precious opal they discovered that precious opal was composed of very constant sized spheres in a tightly packed uniform pattern. In common potch the spheres are jumbled together in no particular pattern and are of different sizes.

Color is produced by light entering the opal and being broken up into colors by the spheres and spaces between them. It follows that the space between the spheres must he uniform since the spheres are uniform. This light phenomenon is called diffraction.

The size or the spheres determines the color one sees. Large spheres (0.00003 mm - equals one millionth of an inch) produce reds. Blue and greens are produced by medium size spheres. Small spheres produce violet.

There is usually not one constant pattern over the whole stone. There are breaks in the size of the spheres or the orientation of the lines of spheres. These breaks in the alignment of the spheres account for the different patterns of opal.

Cutting Australian Opal

Equipment

  • White base-220 diamond - can use 100 grit to flatten back.
  • Crystal - 320 or used 260, front and back.
  • Andamooka jelly, Yowah & some others - 180 grit.
  • Boulder - Iron stone matrix - 180; opal on matrix, worn 220 or 320.
  • Flat lap vs. wheels.
  • All new diamond wheels or flat-laps should be broken in with agate before using on opal.

Trim Saw

  • .010 or .012 thickness of blade, depending on experience.
  • If experienced use 4 inch .004 inch - less waste.

Opal Classes

  • Crystal opal rough much more expensive than "potch & color" or white base.
  • Crystal mostly all color with only a little potch or unwanted inclusions.
  • Crystal harder, more brittle than white base.
  • White base (or potch & color) less color than crystal with potch intermixed with color. Usually 5½ hardness on MOHs hardness scale.
  • Crystal being harder than white base must be cut with more patience, finer diamond wheel. Can be 6 plus on MOHs hardness scale.
  • Crystal easier to orient. Less chance of fractures or unwanted inclusions in rough.
  • Can take more liberties cutting white base. Doesn't shear off usually. There are chances of fractures in rough. You will get less stones out of white base or potch & color.

Routine for Cutting and Cabbing Opal

  • Check edge around all of stone. Grind off thin edges, cracks, and potch, anything that won't be part of finished stone. This step will make orienting stone easier.
  • Orient stone. Decide which side will be top, which side will be bottom. Your cuts should be in same direction as color lines.
  • Flatten bottom. Orient bottom with color lines.
  • Finish shaping stone. (Free form or calibrated.)
  • Smooth bottom edges of stone.
  • Dop stone. When dopping stone make certain stone is level on all sides of dop stick. If not level cutting will be difficult.
  • Make first cut entirely around stone. Distance of cut to the bottom of stone should be the same all around stone. Amount cut off depends on height of dome.
  • Make second cut if necessary. On high domed stones second cut will be necessary.
  • Cab or dome stone. Use twirling motion for getting right slope on stone.
  • Smooth sides, coordinate with dome.

Note: Adjustments of sides of stone may be necessary when cutting stone (to remove fractures, potch or unwanted inclusions). Don't forget to re-smooth bottom edges of stone after removing from dop stick, after polishing stone.

Polishing your Australian Opal

  1. 600 grit diamond (paste or 6oogrit polishing wheel). This grit will remove most small scratches or pits. Sometimes it is necessary to go down to 325 grit. If it doesn't smooth stone it might be necessary to re-cab with diamond wheel or flat-lap. Many times fractures or imperfections' didn't show until 600 grit is used.
  2. 1200 grit polishing diamond. (Paste or diamond polishing wheel) This grit can remove very fine scratches. **
  3. If it doesn't, you will have to go back to 600 grit or 325 grit. .After; 1200 grit used stone should have no scratches or imperfections.
  4. 8000 grit. (Paste or diamond polishing wheel.) This is a polish and will not cut stone.
  5. 14,000 diamond grit. (Paste or diamond polishing wheel). If cerium oxide on leather is available try going from 8,000 to cerium oxide. If not available use the 14,000 polish. Do not use a finer grit as the oil suspending the finer grit may be absorbed by the stone.
  6. Remove stone from dop stick with single edge razor blade. Clean back of stone of dop wax, etc. with worn 600 grit belt. Recheck edges of back for smoothness.

*A twirling motion should be used for all pre-polishing and polishing.

**Fine scratches can be fractures or can be caused by diamond wheel used in cabbing stone. If you have broken wheel in by cabbing a couple agates you shouldn't have protruding diamond problems.