A Faceter’s Guide to Rough Gem Stress
Gem stress can cause stones to break during faceting. Learn what techniques may reduce the odds of fracturing and how to spot problem rough before buying.
9 Minute Read
What Causes Gem Stress?
Many things can contribute to stress in rough gemstones. Heat treatments can cause inclusions in gemstones to explode, creating internal stress fractures. For example, commonplace corundum heat treatments can sometimes create halo-like stress fractures around inclusions in rubies and sapphires. Of course, explosives used in mining can also stress crystals.
The natural growth of crystals can also introduce stress. For example, traces of chromium or vanadium in emerald not only give it its amazing green color but also weaken its crystal structure. Some gems, like tourmalines, show beautiful color zones. However, these zones formed when the conditions of crystal growth changed, sometimes violently.
Into Each Faceter's Life Some Broken Gems Must Fall
Some faceters believe you can somehow relieve gem stress and keep the stone from cracking during cutting. Some may disagree with me, but I think this is a myth. In my personal experience, if stress exists in a crystal and you try to alleviate it by cutting into it, through it, or near it, you're only going to let the stress loose, so to speak. In some cases, this will actually create the flaw you wanted to avoid.
Of course, some people have all kinds of theories to stop gem stress from causing problems. For example, some recommend letting the stone sit in water on the lap. Personally, I think this will only accomplish two things - the stone and the water/lap will have the same temperature, and you'll get wet.
Now, thermal shock is very bad for a stone, so this isn't a bad idea. However, it's only worthwhile if a significant temperature difference exists between the stone and the water. Otherwise, you're wasting your time. I don't think letting a rough crystal sit in water will affect gem stress one way or another. If you want to try it, go ahead. It probably won't make anything worse.
Can Any Cutting Techniques Help Mitigate Gem Stress?
Based on my own experiences, I think the following recommendations may help in some cases.
Use a Fine Lap
By using a fine lap on a stone (such as a 1,200 grit diamond), you're not subjecting it to harsh grinding. Thus, this might help. A fine lap might reduce the odds of making stress flaws travel and get worse. However, it still can't do anything about stress contained in a stone. If you cut into the stressed area, fine lap or not, the stone will still have a flaw
Skin the Crystal
Sometimes, gem stress comes from the outside or "skin" of the gem, so to speak. In such a case, "skinning" the crystal first to get rid of rind cracks could help. However, if the stress doesn't come from the skin or rind, or it's not restricted to the skin, skinning will likely make no difference.
You skin a skin crystal down its length evenly on a fine lap. Skin evenly all the way around.
This may help relieve stress in some cases because some gems have differential stress between the rind and heart. Think of the rind as exerting force to keep the crystal's heart in shape/line (for lack of better words). If you just relieve the stress in one spot, you increase the stress in another area. This will often cause a break. However, if you skin the length all the way around before you start cutting, the stress is gone.
Improving the Odds
If you think your rough has stress problems, I recommend skinning before you start dopping or faceting. Dopping, especially with wax and heat, will often aggravate stress in a crystal. While skinning might help, keep in mind that doing it doesn't mean the gem won't break. However, if the gem has enough stress to break, I'd rather have it break before making a significant time investment.
Essentially, you can sometimes improve the odds that your stone will emerge intact from cutting if you use a fine lap or skin the crystal. However, if the gem has enough stress, it will break. There's just not a lot we faceters can really do about it.
Tourmalines and Gem Stress
Stress doesn't always cause a crack or flaw. It often cuts out with the faceter unaware of it. Nevertheless, some gem types do have stress that will usually cause problems. The stone people associate most with stress is tourmaline. Not only bi-colored stones but also indicolites (blue tourmalines) frequently cause difficulties for faceters.
Most tourmaline usually cuts well with no problems. However, all color varieties can potentially have stress.
Types of Stress in Tourmaline
I've encountered two main types of gem stress in tourmalines. (Other types may exist, of course).
Generally, you can easily spot and deal with this type of stress. Rind stress usually occurs when the gem's outside "rind" forms at a different time, under different conditions, or from a different material than the interior crystal "heart." This type of rind will often have a different color than the rest of the gem. (Think of it like an M&M's. The outside rind is often, but not always, a different color than the inside).
You may encounter a gem crystal with a solid heart but a cracked rind due to stress or a rind that cracks when you cut it. For example, some Afghan tourmaline nodules are crystal centers with the rinds worn off. Sometimes, a piece of the rind remains on the crystal.
This type of gem stress causes the most problems because it's difficult to detect. Sometimes, the rind holds together a crystal that looks flawless. When you cut into the rind, however, the internal stress pops the crystal into several pieces. Unfortunately, there is little you can do about this type of stress. Just let the crystal split and try to cut the smaller pieces.
Occasionally, tourmalines have both rind and internal stress at the same time. I've personally removed cracked rinds from stones, only to have the crystal heart split. Although fortunately not common, tourmalines like this do exist, and you might encounter them. Again, faceters can do little about these stresses except hope they don't cause problems during cutting.
What to Watch for When Buying Bi-Colored Tourmaline Rough
While you can't really tell beforehand if a tourmaline has a stress problem, you can watch for some potential signs.
Bi-colored tourmalines will often break apart at the color lines or separations. Stress exists at these areas. These color zones resulted from alterations during the crystal's formation, such as heating or chemical variations in the underground cavity. The sharper and more distinct the crystal's color zones or separations, the more extreme the stress of the alteration. The environmental changes that led to the sharp color zones occurred more quickly. On the other hand, more gradual color separations may indicate less gem stress, since the environmental changes occurred more slowly.
Some mines are known to be more likely to produce stressed crystals than others. Learning which ones just takes experience.
Some color combinations seem more likely to indicate stress than others. For example, Afghanistan produces a tourmaline with a turquoise blue to pink combination. I've almost never seen rough with these colors without stress flaws at the color separation. Although beautiful, this combination almost never occurs clean and just proves difficult to cut.
Do Research at Gem Shows
When you attend gem shows, particularly shows like Tucson where bi-colored tourmalines abound, pay attention. Look at the finished gems and note the color combinations. This may serve as a good indication that rough in those colors may cut reasonably well. Ask about the sources and mines that produced those gems, too.
What to Watch for When Buying Indicolite Rough
Again, some mines tend to produce more stressed material than others. However, much depends on what has been done to the rough. In the case of blue tourmaline, more often than not, miners heat it in an attempt to "improve" the blue color. This happens even with material that starts with decent to good color anyway.
Why Does Heating Cause Gem Stress?
Heating causes problems in some crystals because basic material differences exist between the rinds and hearts of many tourmalines. They have different levels of stress and react differently to heating and cooling. On the other hand, those tourmalines with no rind/heart differences heat more easily. This also explains why heating faceted stones generally succeeds more often than heating rough. The rinds have been removed before heating.
Why Does Heating Occur at the Mines?
Most likely, the miners take a chance and hope to win the lottery with a Paraíba color or something close. The enormous price differences that top-color blues make encourages the heating of indicolites. Of course, this generally doesn't really improve the color. More often than not, this just damages nice material. Many times, these treatments take the temper out of the crystals. Unfortunately, stress and temper changes due to improper heating don't show in the rough crystals.
I've had many arguments with miners I know about this practice. "Don't heat it. I want untreated, natural colors and will pay for it." I've yet to win one of these arguments. Heating is just a very widespread, common practice with indicolites in particular.
Signs of Heat-Treated Indicolite
Once heating creates stress and flaws in the rough, you don't have a lot of options for dealing with this. Therefore, look for signs of heating before you buy.
Unfortunately, most of the signs, like subtle color differences, take experience to recognize. Look for burned earth or blackened particles in the cracks or ends (particularly matrix ends) of the rough. This usually, but not always, indicates heatings. Types of flaws can also indicate heating, but determining that, again, takes experience.
So, how can you buy quality blue tourmaline rough to cut? The best advice I can give is buy from reputable dealers who know what they're doing and sell quality rough.
A deep blue, unheated indicolite crystal and a matching 1.54-ct gem. Indicolite rough and cut set, Neuschwaben (Neu Schwaben), Karibib District, Erongo Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Do Synthetics Have Less Gem Stress?
While synthetic tourmaline is difficult to manufacture and rarely appears commercially, what about lab-created options for other gems? Some people believe synthetic gem materials are easier to cut and stress free. However, any experienced faceter who cuts synthetic material will tell you that's not always true. Some batches of synthetic rough have stress-related problems, too.
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