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#1
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Fracture question - newbie
When looking into a say, a cabochon-cut amythist and you see wall-like
divisions, is this fault called a fracture ? Is there a site which shows pics of faults etc ? TIA Pete |
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#2
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I'm no expert but there can be structures/inclusions called veils,
curtains, thumbprints, tubes, breadcrumbs, etc. Check www.gemsociety.org www.yourgemologist.com www.rockhounds.com for possible books and online assistance. Carl Cumberpach wrote: When looking into a say, a cabochon-cut amythist and you see wall-like divisions, is this fault called a fracture ? Is there a site which shows pics of faults etc ? TIA Pete -- to reply, change ( .not) to ( .net) |
#3
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On Fri, 01 Jul 2005 18:20:18 -0700, in |õ "Cumberpach"
wrote: When looking into a say, a cabochon-cut amythist and you see wall-like divisions, is this fault called a fracture ? Is there a site which shows pics of faults etc ? TIA Pete Cabochon or facetted or other cutting style, or rough uncut condition has no bearing on the nomenclature, so this applies to all. There are, as Carl points out, many types and variations on inclusions or flaws or internal features of a crystaline gem material (or the non-crystaline ones too.) But of those that will resemble a fracture, there are a few worth mentioning. First, if it's an actual break, or crack, or fracturer (call it what you wish), then there are two principal types worth distinguishing. First is just that, a crack or fracture, or whatever you wish to call it. They follow the directions determined by the stresses that created them, be they damage or just during formation of the gem. There are several common terms for these. "feather" is a common one, especially in diamonds, since they often appear as white feather shaped things in the diamond. They may also simply be called fractures, cracks, or the like. The second is also a full crack or break of this sort, but it occurs in those materials where there are one or more specific directions where the material is especially likely or easy to break. These directions are called cleavage planes, and this type of break is called a cleavage. Some examples of stones where this is possible are diamond (four possible directions), Topaz (one such direction), fluorite (also four directions), Calcite forms, and a number of others. Amethyst does not have any cleavage planes. Cleavages generally differ from other types of fractures in that they follow a flat plane, while most other fractures wander in curved or splintery or other random directions instead of being dead flat planes. Cleavages are generally more serious in a gem because they represent a break that has already started, in a direction that is substantially easier to break, so a cleavage presents generally a significantly higher risk of being extended further into the gem. With both ordinary fractures or cleavages, if they form while the crystal is still forming, it can be possible for the crystal growth process to partially heal the break. Often this leaves fracture or cleavage shaped planes or curves that now consist not of a single fracture, but of multiple small inclusions, sometimes bubbles or tiny residual fractures, or the like. These can resemble whisps of gauze floating in the gem, and are often called veils. They are still a source of weakness in the gem, but considerably less of a risk than a full contiguous fracture or cleavage. Then there are two other common phenomena which can fit your description. Many gem crystals are formed under somewhat less than ideal situations for a uniform crystal. Chemistry in the zone may change, as the stone froms, such that color from one part of the gem can change or vary. Sometimes the different colors or intensity of colored areas in the crystal can be quite sharp demarkation zones, which can appear as an internal "surface" or plane with no break or fracture, but clearly seperating one colored area from a differntly colored, or differently intense area of the same color. This is called color zoning. A dramatic example of this might be Ametrine, where yellow citrine and purple amethyst can exist in the same crystal, often clearly seperated by a sharp demarcation between colors. Tourmaline too, is famous for this, with the so called watermelon trourmaline showing red and green, and sometimes clear zones all in the same gem. And finally, as crystals form, sometimes the atomic orientation of the crystal structure can appear to switch. Often this is the result of two conflicting crystal orientations trying to form at the same time, or two crystals intergrowing together (sorry, that's a poor explanation) Either way, what results is called a twinned crystal, so that the single stone switches it's atomic structure or symmetry directions, usually along a planer direction in the stone. Sometimes it will do it repeatedly as the crystal grows, so the end result looks like a stack of layers. Because the optics of a gem can vary according to the direction through the crystal, the plane along which this change from one twinned direction to another occurs, can sometimes be visible as a distinct plane in the stone, sometimes even a slightly reflective one. Corundum (sapphire, ruby) is particularly prone to this, giving rise to visible striations in the stone The phenomenon is also responsible for some of the interesting visual effects in labradorite or moonstone type feldspars, among a number of others. These visible twinning planes can look like fractures, but are not actual breaks. Hope that helps. Peter Rowe |
#4
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try
1) uni of california berkeley: (a real gem of a site) http://ist-socrates.berkeley.edu/~eps2/wisc/image.html gallery of gems http://ist-socrates.berkeley.edu/~ep...emologist.html (ask a gemmologist) http://ist-socrates.berkeley.edu/~eps2/ (Gems & Gem materials course) http://ist-socrates.berkeley.edu/~eps2/wisc/jpeg/ index of pics (lots of goodies - have your pick) 2) if you have access to a good (reference) library try Photoatlas of Inclusions in Gemstones ~E.J. Gubelin , J.I. Koivula (best book on subject) for info see also http://www.ganoksin.com/jewelry-book...3855040958.htm 3) try search google images 'inclusions gems' I hope this helps regards norwick |
#5
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Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain'
best describes the flaw in the stone. The ring has gone back for exchange or refund as there was also a surface chip 1mm x half mm. I assume the best quality gems get the fancy-cut and the 'swag' gets the cabochon treatment. Pete |
#6
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On Sat, 02 Jul 2005 17:13:19 -0700, in ¸õ "Cumberpach"
wrote: Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain' best describes the flaw in the stone. The ring has gone back for exchange or refund as there was also a surface chip 1mm x half mm. I assume the best quality gems get the fancy-cut and the 'swag' gets the cabochon treatment. Pete Swag isn't quite the word I'd use, since the lower clarity grades of amethyst can still be very pretty, just less expensive. You're right that the very clean stones are more commonly reserved for faceted cuts, where larger inclusions would mess up the optics of the stone. Cabochons generally don't need exceptional clarity to take advantage of optics, since there isn't generally an issue of light needing to enter the stone and reflect back out unimpeded. In fact, some inclusions can simply make the stone look more individual and unique, if you can choose the approach the issue with that mind set, instead of simply lumping it all in the "flaw" catagory. Some inclusions can be quite fascinating and beautiful. The simple truth is that inclusions, or flaws, or whatever you wish to call them, are part of the way mother nature makes gems. They are the signature and fingerprint of the formation process, and if you crank the magnification up high enough, there's virtually always something in any stone, somewhere. But of course it's your money, and if you prefer the look of a very clean stone (and many people do), then you should not settle for less than what you wish. But do understand that many commercially produced lines of jewelry using such cabochon cut inexpensive stones are priced on the assumption that the stones will not be totally clean. Insisting on a clean one may in some cases be asking for a ring better than what the seller is normally selling, or has priced the item at. With amethyst, for example, there can be a significant difference in cost between a parcel of stones that are eye clean, versus a parcel of the same color that have visible veils and inclusions, since the latter are far more common with amethyst cabs. Be prepared, in such cases, to pay the difference in cost between the stock quality, and the higher clarity one, if that is the case with your seller. This issue is, by the way, one big disadvantage to buying over the net or via mail order, as opposed to in person in a traditional store, where you can carefully examine the stones before buying. On the net or from a catalog, one often is seeing a photo of a generic example of a piece, rather than the actual one, and even if it's the actual piece, gems are hard to photograph in such a way that the inclusions show up realistically. Plus, photographers understandably don't go out of their way to make inclusions more visible than needed when doing product photography... Peter |
#7
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Though often lower cost goods also get rough treatment and might be
thrown into a tray or other container to bang against each other, I feel the surface chip is the major reason I'd be inclined to return/exchange the item. As Peter implied, a cab of amethyst with inclusions and fractures (that do not otherwise put the stone at increased risk of poor durability) can still be beatifully colored and unique in appearance. If you have the inclination, joining a local Gem & Mineral club may be interesting to you. You can learn to cut and polish your own stones and have them set or learn metalsmithing and set your own stone. Soemtimes there are colleges offering such courses as well. Carl Cumberpach wrote: Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain' best describes the flaw in the stone. The ring has gone back for exchange or refund as there was also a surface chip 1mm x half mm. I assume the best quality gems get the fancy-cut and the 'swag' gets the cabochon treatment. Pete -- to reply, change ( .not) to ( .net) |
#8
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Peter W.. Rowe, wrote:
On Sat, 02 Jul 2005 17:13:19 -0700, in ¸õ "Cumberpach" wrote: Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain' best describes the flaw in the stone. The ring has gone back for exchange or refund as there was also a surface chip 1mm x half mm. I assume the best quality gems get the fancy-cut and the 'swag' gets the cabochon treatment. Pete Swag isn't quite the word I'd use, since the lower clarity grades of amethyst can still be very pretty, just less expensive. You're right that the very clean stones are more commonly reserved for faceted cuts, where larger inclusions would mess up the optics of the stone. Cabochons generally don't need exceptional clarity to take advantage of optics, since there isn't generally an issue of light needing to enter the stone and reflect back out unimpeded. In fact, some inclusions can simply make the stone look more individual and unique, if you can choose the approach the issue with that mind set, instead of simply lumping it all in the "flaw" catagory. Some inclusions can be quite fascinating and beautiful. The simple truth is that inclusions, or flaws, or whatever you wish to call them, are part of the way mother nature makes gems. They are the signature and fingerprint of the formation process, and if you crank the magnification up high enough, there's virtually always something in any stone, somewhere. But of course it's your money, and if you prefer the look of a very clean stone (and many people do), then you should not settle for less than what you wish. But do understand that many commercially produced lines of jewelry using such cabochon cut inexpensive stones are priced on the assumption that the stones will not be totally clean. Insisting on a clean one may in some cases be asking for a ring better than what the seller is normally selling, or has priced the item at. With amethyst, for example, there can be a significant difference in cost between a parcel of stones that are eye clean, versus a parcel of the same color that have visible veils and inclusions, since the latter are far more common with amethyst cabs. Be prepared, in such cases, to pay the difference in cost between the stock quality, and the higher clarity one, if that is the case with your seller. This issue is, by the way, one big disadvantage to buying over the net or via mail order, as opposed to in person in a traditional store, where you can carefully examine the stones before buying. On the net or from a catalog, one often is seeing a photo of a generic example of a piece, rather than the actual one, and even if it's the actual piece, gems are hard to photograph in such a way that the inclusions show up realistically. Plus, photographers understandably don't go out of their way to make inclusions more visible than needed when doing product photography... Peter Personally I enjoy working with included amethyst. You can do beautiful things cutting free-form cabochons to use the various 'flaws'. --RC |
#9
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"Carl 1 Lucky Texan" wrote in message If you have the inclination, joining a local Gem & Mineral club may be interesting to you. You can learn to cut and polish your own stones and have them set or learn metalsmithing and set your own stone. Sometimes there are colleges offering such courses as well. Where I live, was heavily mined, up till the end of the 19th Cent., tin and copper chiefly. There's a disused mine (open to the public) with a very cheap gem shop, which sparked my interest 3 years ago, when I moved here. I have even found some low-grade amethyst in the back garden. If there's no gem club local, there may be one in Plymouth (25 miles East). Hmm, worth a thought. |
#10
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"Peter W.. Rowe," wrote in message "Cumberpach" wrote: Thanks to Peter, Carl and norwick for the info. I suspect 'feather curtain' best describes the flaw in the stone. The ring has gone back for exchange or refund as there was also a surface chip 1mm x half mm. I assume the best quality gems get the fancy-cut and the 'swag' gets the cabochon treatment. Pete Swag isn't quite the word I'd use, since the lower clarity grades of amethyst can still be very pretty, just less expensive. You're right that the very clean stones are more commonly reserved for faceted cuts, where larger inclusions would I should have mentioned the fact that I purchased the item on one of those falling price tv channels. The ring shown by the presenter and the ring in their light-box were of very high quality. They have a 10 day money back policy and they are trying to exchange it, so I can't complain too much - it's just the hassle factor. |
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