CompuServe Thread

Frank… —> I have cut some pretty decent stuff but really wonder if I am doing it right and if I'm getting the best out of my rough. I have never talked to another opal cutter besides dealers. First off, I can sympathize (empathize?). It sounds like you've followed the same path I have. I started by digging opals at the Barnett Mine in Opal Canyon north of Red Rock in about 1987. I'd say I worked up about 200 stones, most very small. This with almost no background (and certainly no training) in lapidary except having a 3# tumbler for the agate and jasper I was finding in the Mojave desert. I read a few books and went from there with trail and error. It bears mentioning that I also have background making stained glass windows. Kept myself out of the w*** force for about two years doing custom commissioned projects out of my garage. Of course I didn't realize at the time how much that experience was going to help me in working with opal, but since opal is very similar to glass both in structure and behavior I discovered I bring to opal a pretty fair understanding of how to cut and set stones as a result. I have cracked two opals in the setting process to date and in both cases I was effectively trying to push water up a rope. OK, enough on my background for the moment. Here's a few items that might need to be addressed… 1) selection of rough: -this should always be a reasoned decision based on an estimate of the value of finished stones. Paul Downing describes it as "cutting your way out of a parcel". The equation includes the weight of the parcel, the weight of finished stones (rule of thumb is 20% for Australian), and the value (quality level) of the resulting stones; and for myself, I add the value of the time it takes to do the work. These are all estimates obviously and the time factor can be quite variable. I fully polish the backs which virtually doubles my cutting time but I do it for a couple of reasons. Mostly for the love of the stones but also because I believe it ensures the least chance of causing a crack to start in the setting process (has to do with the opal behaving like glass). Others assert that leaving the back unpolished can enhance the play of color by reflecting some light that would otherwise escape through a polished back. I defer to their wisdom, it may be true and if I ever polish the back and find the stone to be less beautiful I will get out some 600 grit paper and roughen the back again (g). There was a member of the AOS several years ago, now deceased, who had developed mechanical cabbing equipment and I'm told he was able to cut calibrated finished stones in batches that averaged ten minutes per stone. -Tim Thomas of House of Tibara insists that any fair dealer will "window" a parcel of rough for a potential buyer. Windowing is to grind a small area to make the color bar visible. -always examine rough dry…look for cracks, candle it like an egg for included sandstone etc. Is the play of color thick enough and level enough to cut to a nicely domed cab? If the color bar is wavy, do you have the carving skills/equipment to follow it? (This can be the subject of a whole 'nother message in itself.) A good dealer will be able to provide commentary on the location the rough came from and the depth of the mine. Depth of the mine is asserted to be significant because it is believed that opal from deeper mines has a greater tendency to be "cracky" as it may have a higher water content and thus self-destruct as it dries. Personally I'm not convinced this is true, again I defer to the wisdom of others. However, I suspect the reason some opal cracks has more to do with the structural formation process; referring here to stained glass again…handblown glass is much more likely to contain internal stress flaws than manufactured glass. Experience is really the best teacher here and it is often recommended that one maintain a notebook detailing each parcel's history and results. 2) cutting for best results: -most dealers try to give the buyer every opportunity to choose rough. An honest dealer should also allow a buyer to return a parcel within a reasonable length of time if it has not been altered. But once you start cutting, it's yours. -Ironically, the higher the quality of rough the easier it is to get results. Medium to poor rough is usually challenging in many respects. The color bars may be thin and/or wavy. The number of different colors showing may be limited. The potch layers may constitute a great percentage of the material making it tough to estimate the resulting finished weight of stones. The 20% guide above might need to be dropped to 10% or in some cases even less. OTOH, a piece of rough which shows color throughout may yield as much as 30 to 50% finished stones, perhaps even more. -always spend some time examining a piece before beginning to grind. Look at it not only from all angles, but even more importantly IMO in varying lighting conditions including in shade and shadows. I have finished stones that just "scream" when there is almost no light and which only look OK in full sunlight. Others are brilliant in flourescent compared to natural light. These are exception of course but it all has to do with the matrix of silica spheres in a particular stone. -deciding how to orient rough involves knowing what constitutes highest value in finished stones. Color pattern and brightness of fire are most important, everything else being equal. To get the most out of rough you need to look for the orientation that best enhances these two characteristics; generally, this is true even if it means cutting smaller stones. 3) valuation of finished stones: -the AOS has an evaluation system which attempts to systemize the process for most commonly available finished opals. The approach is three pronged and begins with determining the base color of the stone. Then the play of color is assessed, looking for number of colors and predominance of any particular color with red multi-color being most valuable. Brightness, or intensity is assertained by comparison to a set of "standard" stones. The AOS system then applies an element termed "Appeal Factor" which is an assessment of a number of other considerations, most importantly type of play of color but also includes such features as shape (sadly calibrated generally is more valuable than baroque which often means a lot of nice material is ground away by "commercial" cutting), size, height of dome, the particular stone's overall appearance, and how much of the surface has color. The AOS has evaluation kits consisting of carefully selected "standard" opals to be used in conjunction with the above process. -Paul Downing has also produced a system which he presents in his book on opal valuation. Knowing something about valuation has helped me immensely in deciding how to orient and cut particular pieces of rough. There are oocasions when I realize part way through the cutting process that a particular stone needs to be cut at a slightly different angle. I then re-dop and adjust the orientation accordingly. One thing I found frustrating at first was the relluctance of everyone, even dealers, to comment on the specific value of any of my finished stones. I now understand that the "pricing" of opal is nebulous at best. I have a personal rule in this regard…if someone buys your opal your price was too low 8-). Actually, this is a moot point in my case since I don't sell my finished stones. I'm still at the stage where I love them too much to part with them (g). Now, Frank…and anyone else who has gotten this far here… If any of the above has prompted questions or thoughts about how best to achieve a good yield from a parcel of rough opal, fire away. -Russ