Stress Sensitivity in Roughs
Does sensitivity (stress) exist in rough?
The answer is yes it does in some rough. But one of the major myths that people seem to believe is that you can some how relieve the stress sensitivity in a crystal and keep it from cracking.
I realize that there are people that will not agree with me. But in my personal experience if there is stress in a crystal and you relieve that stress by cutting into it, through it, or near it. The stress is going to let loose and in some cases going to create a flaw.
Note: There can be many kinds of stress, often there is man-made stress in crystals from heating or dynamite, besides just the natural stress that can occur during crystal growth.
Everybody looses a stone once and a while. Here is an example… This stone was quite valuable and just split with no warning after cutting. Everybody looses a stone once and a while
That is how life is. Yes, I know there are people with all kinds of theories to stop stress from causing a problem. Here is one. Letting the stone sit in water on the lap – The only real thing this would do is make the stone and water/lap the same temperature (and maybe get you wet). Which is not a bad idea, thermal shock is very bad for a stone. But this is only worth doing if there is a significant temperature difference between the stone and the water. If there is no temperature difference (there usually is not much difference) or just a minor one then personally I think you are wasting your time. I do not think letting the crystal sit in water will change the stress one way or the other.
Some theories do not work in my experience, but then again if you think they do and want to do them. It does not hurt anything. What ever works for you.
Here is what I know works or at least helps.
Use a fine lap
Not a bad idea, it may help a little bit because by using a fine lap the stone in question is not being subjected to harsh grinding. However, in my experience while a fine lap might reduce the odds of making stress flaws travel and get worse. A fine lap cannot do anything about stress contained in a stone/crystal and if you cut into the stressed area, the stone is still going to have a flaw, if it is going to.
Skin the crystal first
It depends on the stress in the crystal, some times the stress is on the outside of the stone in the skin. In this case “skinning” the crystal to get rid of the rind cracks (if there) is a good idea or if you think there is stress in the crystal. However, if the stress is not in the rind, or not only in the rind. Skinning the crystal will often not make a bit of difference.
I do recommend skinning crystals that you think may have stress problems. In my experience… in some cases skinning the crystals will help. Skin the crystals down their length evenly on a fine lap, skin evenly all the way around. I think the reason skinning the crystal helps in “some” cases is that there is a differential stress between the rind and heart of the crystal(s). Think of the rinds as exerting force to keep the heart of the crystal in shape/line (for lack of a better word). If you just relieve the stress in one spot you increase the stress in the area often causing a break. If you skin the length all the way around before you start cutting the stress is gone. Remember skinning a crystal does not mean it will not break, but it often can help and also I think that if the stress there is great enough to break the crystal I would much rather have the break happen before I start cutting and/or dopping.
Note: Dopping, especially with wax and heat will often aggravate stress in a crystal, so it is always best to relieve stress in the crystal if you think it may be present, before dopping.
My opinion is. If the stress is there and you relieve, or change it in some way by cutting. The stress is going to come loose and will often create a flaw or problem in the stone, if the stress is bad enough to do that. Not a whole lot we faceters can do about it really. Some times we can improve our odds by skinning a crystal or using a fine lap. But…
Note: Stress does not always cause a crack or flaw, often it cuts out with the faceter not even aware it was there. But in some crystals stress typically can cause problems.
Tourmaline is probably the stone that most people associate with stress. Not just bi-colored stones but also Indicolites are commonly a color that people have problems with.
Note: Most Tourmaline usually cuts well with no problems, but all colors can “possibly” have stress.
There are basically two main kinds of stress in Tourmaline in my experience. Of course there can be and are others but I generally see theses two stress types.
This type of stress generally is easy to spot and deal with. Rind stress is usually when the heart of a crystal is covered with an outside (rind) of material that is formed at a different time or often another material/color. The rind is often a different color. Think of an M& M piece of chocolate candy. The rind is on the outside and is often a different color (but not always) and is often cooled, heated, or a different material make up than the center. What you end up with is a “heart” of the crystal that is usually solid and a rind that is cracked, or becomes cracked when you cut into it. For example look at some Afgahan Tourmaline nodules, these are the centers of the crystals that the rind has worn off of. Some times you will see a piece with part of the rind still on.
This type of crystal is the one that causes the most problems. Often the crystal is being “held together” by the rind and looks flawless. It is flawless until you cut into the rind and the internal stress pops the crystal into several pieces. There is not much that you can do about this and the stress is often not apparent. Just let it split and try to cut the smaller pieces. This is what I meant when I said… ” If the stress is there and you relieve, or change it in some way by cutting. The stress is going to come loose and will often create a flaw or problem in the stone.”
Both (double whammy)
There are occasional crystals that exhibit both these types of stress at the same time. I have had crystals that had rind cracks and when I cut the rind off, the heart of the crystal split in completely different places. These crystals are not common thank God, but they are possible. As a faceter there is nothing that we can really do about the stress already in the crystal, except hope that it is not there or not going to cause a problem during cutting. While it is not really possible to know ahead of time if a piece of Tourmaline or bi colored crystal is going to have a stress problem.
There are a few basic things to watch for. Bi colors Tourmalines often have stress in the crystals and will occasionally break apart at the color lines/separations. The main reason for this problem is that the crystals have stress at these areas because of dramatic changes that took place during crystal formation. When the color changed, the crystals under went a significant change, usually heat, and solution (different color).
The sharper and more distinct the color zones or separations in the crystal, the more extreme the stress that is likely to have taken place (the faster the color changed and the environment changed during crystal formation). In other words a gradual color change is usually more stable and less stressed because the forces that created it were more gentle and gradual.
Different mines are known to be more likely to have crystal “stress” than others. Knowing the mines and the ones more likely to have stress problems just takes experience. For example most of the Nigerian Rubellite mine’s bi colors and material in general seem to be pretty stable. I have cut a lot of them. But there are a few other mines that almost always produce material that has stress and flaws.
Some color combinations seem to be more stressed than others. For example Afghanistan Turquoise to pink is a combination I almost never see even rough with out stress flaws at the color separation. This combination is just difficult. Beautiful, but almost never clean and cuttable.
Look and see in the cut stones, particularly at a gemshow like Tucson (Feb.), which bi colors are around and which ones are clean. Seeing the bi colors already cut and clean is a good indication that the rough in question will cut reasonable well.
What about Indicolite?
Well again it does depend on the mine. But it also depends on what has been done to the rough. In the case of blue Tourmaline more often than not the miners have heated it trying to “improve” the blue. Miners often try to heat and “improve” the colors of blue Tourmaline, even material that has a decent to good color to begin with.
Note: I suspect the heating often causes a problem in some crystals because there are some basic material/mineral differences between the rind and hearts of many Tourmalines. The rind and heart are different and have different heating/cooling and stress properties. That also explains why some Tourmaline heats easily (no rind heart differences) and why in general heating is more successful after the stones are faceted (the rinds are gone).
Why? Well they are hoping to win the lottery and get “Paribia” color or some thing close. The main reason they try heating is the enormous price that the top color(s) bring. Of course they do not generally improve the color and they often damage nice material. Be aware that many times they basically take the temper out of the crystals, and unfortunately the stress and temper changes due to incorrect heating do not show in the rough crystals.
I have had many arguments with miners I know about “do not heat it, I want it natural and will pay for it…” This is an argument I have yet to win, they still usually heat it. Heating is a very common practice with Indicolite in particular. Often the heating has created stress and flaws in the rough and there is not a lot that can be done about it. So if you see signs of heating beware.
What are signs of heating? Well a lot of the signs take experience to recognize. Color is a sign, the only way to know is experience. Look for burned earth or blackened particles in cracks or ends (particularly matrix ends) of the rough. This is usually, but not always a sign of heating. The type of flaws and the way the crystals work are more signs, but again take experience.
I have cut a lot of Tourmaline and personally really do not notice much difference between blue Tourmaline and other colors, provided the blue has “not been heated”.
How do you buy quality blue Tourmaline to cut? The best advice I can give you is buy from a dealer that knows what he is doing and sells quality rough.
In conclusion, there are some methods like “skinning” that may help reduce the problems with stress and fractures. But remember that to some extent we are dealing with the variables of nature and that from time to time some crystals are going to break. That is just life and there is not a lot that can be done about it.
As a side note for people that may think man-made materials are easier and never have problems. No that is not correct, some batches of man-made materials have the same types of stress related problems too. Just ask any experienced faceter that cuts a lot of man-made materials.