Turning Precious and Semi-Precious Stones

By Oliver Buettner

Precious and semi-precious stones have captured our hearts and imaginations since before bronze age. Through the ages we learned to find them in the remotest of locations and extract them. We probably first found them tumbled in river beds and at the oceans edges. We learned to shape them, carve them and polish them into things of beauty and objects of desire. Like the magpie, I have been attracted to them since I was very young. Studying books on the subject brings to your attention the vast variety of specimens available and the properties they possess such as something known as the Moh's hardness scale. All stones are rated according to the Moh's Hardness scale. (see attached article on hardness).

If you find reputable dealers of rough stone (which is what we want for turning) you won't have to go around scratching the stuff up to figure out what it is. The important thing for us to know is the hardness of the rock because we need to use something harder than it for cutting and shaping. Diamond is the hardest elemental known to man and is therefore a surefire way to cut anything that strikes your fancy. Industrial diamond bits, blades and cutters are available in many shapes and sizes. There are stones that are softer than high speed steel and can be shaped with your turning tools or carbide dremel bits, but not many. Many gemstones have cleavage planes (Like mica which can be pulled apart in layers (along its cleavage plane) and cannot be cut reliably with steel tools even if they are softer than the metal. Every time you contact the piece along the plane (like grain in wood but often running in multiple directions), which is the weakest part of the gem, it will break off. The easiest way to manipulate rock is with a dremel and a diamond blade or bit.

Gemstone shows come to the Chicago area at least twice a year and I have included the name of a reputable dealer of rough to get you started. Find something you like and can afford to ruin in the beginning, just like you did when you started turning and searched for wood. If you know what you are looking for, do some rock hounding and search for them yourselves. Be advised they look much different in the rough than in a polished state. That makes them easy to overlook without study and experience.

I have chosen Turquoise for today's demonstration because of its beauty, desirability, lack of discernable cleavage planes (although there are two), workable hardness (5-6), reasonably affordable cost (for small adornments), light weight and color. (see attached page on the properties of this rock)

All work on cutting rock must be done using water as a lubricating and cooling fluid. Heat will merely discolor some stones, but many will crack, fracture and break if you get them too warm or hot. Water also keeps the cutting dust down to a minimum. You do not want to breathe the dust from cut rock as many are poisonous or cancer causing, if not downright unpleasant. Use the same precautions you should used as when sanding or cutting wood. Eye protection and/or face shield are highly recommended along with a quality respirator. You may want to use an air filtration system as the dust can be very fine and will linger in the air for some time before settling.

TURNING STONE
Choose a piece of rock and wet it. Use a toothbrush to remove the loose material on the outside. Wire wheels can also be used. Mentally divide the specimen to provide the size you need for your project with the least amount of waste. For finials you will be cutting (slabbing) the rock as you would wood, into a turning square. Crystals can be used without cutting (with the exception of possibly the bottom before mounting. Actually for the most part you will work with the piece from beginning to end just as you presently do your wood piece. Merely the tools, mounting, turning methodology and finishing procedure will be different. Fundamentally it is the same procedure. Stone is sanded to the desired finish sheen or texture without the aid of waxes or varnish, etc after sanding is complete.

I like to flatten one side if possible to provide a stable base when cutting rock into sections. The same as if you were going to cut something on a table saw or band saw. If it rocks and rolls while you are cutting it you risk the safety of your fingers. Depending on the hardness you can employ a sanding disc to accomplish this, but you must be able to get it wet as water is required to reduce heat and lubricate the pad (this will also extend the life of the sandpaper).

Once the rock is prepared for mounting this brings about new challenges. It cannot be merely tightened in a 4-jaw chuck as that will damage both rock and chuck. I first tried CA (don't we all) and found that it was much too brittle and my first piece promptly left its mount and went sailing across the room. I tried dopping wax, which is what is used in the lapidary process, but that also proved unreliable. I use a piece of wood about 2-3" long, rounded and the center (end) scooped out 1/8" to ¼". Epoxy is the glue of choice for this procedure. I have yet to lose one to epoxy.

When gluing, attempt to center the specimen and don't tilt it. After the glue has cured I then mount the wood into my chuck. Check for straightness and get ready for cutting. Given water is used in this operation I cover my lathe with a towel to reduce the mess and keep the lathe from rusting. You will need to use the tool rest but not the live center. That is why the wood mounting block is so short, as we don't want to run into deflection problems while cutting and shaping.

Using a dremel and a flexible shaft with the diamond blade, we are ready to round the piece. Using one hand to keep a wet sponge in constant contact with the rock and the other holding the flexible shaft against the tool rest, we're ready to round and shape the piece. The lathe speed should be slow. I leave mine set at 500 rpm for the entire procedure, including sanding.

Once the desired shape has been attained, it's time for everyone's favorite… sanding. For turquoise I have found micro mesh works very well. When I tried a tourmaline piece, Mmesh was insufficient to the task and will take further trial to get the intended result. Lapidaryjournal.com is a useful site for tips on finishing procedures of various gem types. Chat rooms are also a resource (jewelry, lapidary, etc). Each gem type will require a different finishing method, so keep notes once you have a working formula.

Once complete, mount (inset) the gem into your wood turning for a feature that will set you apart from the crowd. Enjoy the learning process and turn some beautiful and exciting stuff.

Turquoise
December birthstone
a greenish-blue fine-grained secondary mineral consisting of hydrated copper aluminium phosphate. It occurs in igneous rocks rich in aluminium and is used as a gemstone. Formula: CuAl6(PO4)4(OH)8.4H2O

Turquoise, hydrous phosphate of aluminum and copper, Al2(OH)3PO4·H2O+Cu, used as a gem. It occurs rarely in crystal form, but is usually cryptocrystalline. Turquoise is opaque and has a waxy luster; the color varies from greenish gray to sky-blue. The sky-blue varieties are the most valued as gems, but because of their porosity they easily absorb dirt and grease and change in color to an unattractive green. Exposure to heat or sunlight is also injurious to the color of the turquoise. The finest specimens come from Iran; other sources are the Sinai peninsula and the SW United States, especially New Mexico, Nevada, Arizona, and Colorado. Turquoise matrix is a rock including fragments of turquoise, cut as a gem stone. Variscite, the hydrated phosphate of aluminum, is sometimes used as a substitute for turquoise. It occurs in crystals of the orthorhombic system and in massive form; minable deposits are found in Utah.

Turquoise is a valuable mineral and is possibly the most valuable, non-transparent mineral in the jewelry trade. It has been mined for eons since at least 6000 BC. by early Egyptians. Its history also includes beautiful ornamental creations by Native Americans and Persians. Its popularity is still quite strong today. Although crystals of any size are rare, some small crystals have been found in Virginia and elsewhere. Most specimens are cryptocrystalline, meaning that the crystals could only be seen by a microscope. The finest turquoise comes from Iran but is challenged by some southwestern United States specimens. Turquoise is often imitated by “fakes”, such as the mineral chrysocolla, and poorer turquoise specimens are often dyed or color stabilized with coatings of various resins. The name comes from a French word which means stone of Turkey, from where Persian material passed on its way to Europe.

Physical Characteristics

Best Field Indicators are crystal habit, hardness, luster, color and associations


Hardness
A good property in mineral identification is one that does not vary from specimen to specimen. In terms of reliability, hardness is one of the better physical properties for minerals. Specimens of the same mineral may vary slightly from one to another, but generally they are quite consistent. Inconsistencies occur when the specimen is impure, poorly crystallized, or actually an aggregate and not an individual crystal.

Hardness is one measure of the strength of the structure of the mineral relative to the strength of its chemical bonds. It is not the same as brittleness, which is another measure of strength, that is purely related to the structure of the mineral. Minerals with small atoms, packed tightly together with strong covalent bonds throughout tend to be the hardest minerals. The softest minerals have metallic bonds or even weaker van der Waals bonds as important components of their structure. Hardness is generally consistent because the chemistry of minerals is generally consistent.

Hardness can be tested through scratching. A scratch on a mineral is actually a groove produced by microfractures on the surface of the mineral. It requires either the breaking of bonds or the displacement of atoms (as in the metallic bonded minerals). A mineral can only be scratched by a harder substance. A hard mineral can scratch a softer mineral, but a soft mineral can not scratch a harder mineral (no matter how hard you try). Therefore, a relative scale can be established to account for the differences in hardness simply by seeing which mineral scratches another. That is exactly what French mineralogist Friedrich Mohs proposed almost one hundred and seventy years ago. The Mohs Hardness Scale starting with talc at 1 and ending with diamond at 10, is universally used around the world as a way of distinguishing minerals. Simply put; the higher the number, the harder the mineral.

Below is the Mohs Hardness Scale:
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum (ruby and sapphire)
10. Diamond

In order to use this scale, it is necessary to have on hand some of the minerals in the scale. If you wish to test an unknown mineral for hardness you might want to start with an ordinary specimen of apatite to see if the unknown mineral can scratch it. If the unknown mineral scratches the apatite, then you can conclude that it has a hardness of 5 or more. If the apatite can scratch the unknown mineral, then the unknown mineral has a hardness of 5 or less. If they can scratch each other, then the unknown mineral has a hardness of 5. You will need to perform other tests to narrow down the hardness. If it is softer than apatite, try calcite, etc., etc until you have narrowed down the approximate hardness. Remember, this is a relative scale and a mineral that can scratch a mineral that has a hardness of 4.5 may be given a hardness of 5, but it still might be softer than apatite.

One word of caution for inexperienced collectors: do not SCRATCH NICE CRYSTAL FACES! A fractured, cleaved or inconspicuous part of the mineral should still give a good hardness test and not damage a potentially wonderful specimen.

What if you do not have the minerals listed in the Mohs Hardness Scale? Well, a collector might keep a few items of known hardness in a "hardness kit"; just in case they are needed.

Below is a revised Mohs Hardness Scale with some everyday items listed:
 1. Talc
 2. Gypsum
        — fingernail at 2.5
 3. Calcite
        — copper (old penny) at 3.5
 4. Fluorite
 5. Apatite
        — window glass or typical knife blade at under 5.5
 6. Orthoclase
        — streak plate or good steel file at over 6.5
 7. Quartz
 8. Topaz
 9. Corundum
10. Diamond

Again, the Mohs Hardness Scale is only relative. Meaning that fluorite at 4 is not twice as hard as gypsum at 2; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond. An absolute hardness scale looks a little different than the relative scale. Using sensitive equipment, a comparison of the absolute hardness of minerals can be measured. It turns out that most minerals are close in hardness. But as hardness increases, the difference in hardness greatly increases as seen in the scale below.


Below is an absolute hardness scale:

      The simpler, relative Mohs hardness scale is
      much easier to remember and use.

It is easy to see why diamond gets so much respect as the hardest natural substance know to man. The next hardest mineral, corundum, is four times softer! There are many substances that are currently being created and studied to beat diamond in hardness. But diamonds' all carbon, extremely dense, structurally sound and tightly bonded structure is hard to beat. At present only diamonds created with isotopes of carbon have exceeded the mark of 10 on the hardness scale.

Hardness is particularly important for gemstones. Very few soft minerals are cut as gems and when they are, they generally are cut only for collectors and not for wearable jewelry. Apatite is one of the softest of gemstones. Mostly gemstones have a hardness of 7 or more. Hardness also plays a major apart in the minerals that are used for grinding, polishing and other abrasive purposes. Soft minerals can be used as high temperature lubricants, pencil lead, talcum powder, paper gloss, etc.

Resources:
lapidaryjournal.com Articles and ideas
www.neweragems.com (supplier)
   (next show at Donald E. Stephens Convention Center - December 12-14)

References

Magazine:
   Lapidary journal
   lapidaryjournal.com

Tool supplier:
   Rio Grande
   1-800-545-6566
   www.riogrande.com

Rough Supplier
   New Era Gems, 14923 Rattlesnake Rd. Grass Valley, CA 95945
   Toll free 1-800-752-2057
   www.neweragems.com

Next Gem Show:
   December 12-14
   Donald E. Stephens Convention Center
   Rosemont, IL

Diamond Cutting bits and blades:
Harbor Freight offers the best deal

Chicago Woodturners 2003
A Chapter of the American Association of Woodturners
Last Updated February 24, 2008