rockhoundoz
starting to spend too much on rocks
Member since March 2017
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Post by rockhoundoz on Jan 14, 2019 21:39:27 GMT -5
Here's a stone I've been collecting in gravel from a location in east coast piedmont country. My immediate thought was quartz, either 'blue quartz', 'blue chalcedony' or even faded amethyst. Then I noticed that the color of the stone changes under different lighting &considered zoisite or iolite. Out of all the stones I mentioned, zoisite is the only one known to occur in this region via mindat. Here are two pictures. I suppose that having the white paper towel under the stones creates 'transmitted light'. Since blue chalcedony is known to be yellow/tan/colorless while having 'transmitted light', but appears blue when lighted from above, I suppose that's evidence for blue chalc. Hardness is around 7 & streak is white to colorless. Any opinions? Again, the lighting in the 2 pics are identical (indirect sunlight), the only difference is white background vs bamboo background. Thanks!
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Post by orrum on Jan 15, 2019 4:55:49 GMT -5
Interrsting, no idea but waiting to find out.
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fuss
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Post by fuss on Jan 15, 2019 10:14:32 GMT -5
I think since the light source is the same the towel is causing the color difference. It looks like Quartz to me. Try a more controlled test without the towel as well as different light sources.
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Deleted
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Post by Deleted on Jan 15, 2019 14:22:52 GMT -5
As fuss said, it is probably the background playing tricks with your camera's sensor. I think chalcedony or stream-eroded amethyst. Zoisite only gets a blue color when heated, and doesn't look like any iolite I've had. As there are no intact crystal or botryoidal faces, you might want to go ahead and tumble or cut them. Attractive pieces!
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gemfeller
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Post by gemfeller on Jan 15, 2019 20:24:33 GMT -5
Looks like chalcedony to me. Mineralogists tell us the difference between chalcedony and agate is that agate is banded and chal. is not. As for color, the only real test is in natural light with a neutral background. It's hard to judge from your images.
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Post by Lapidaryrough / Jack Cole on Jan 15, 2019 21:50:30 GMT -5
Looks like chalcedony to me. Mineralogists tell us the difference between chalcedony and agate is that agate is banded and chal. is not. As for color, the only real test is in natural light with a neutral background. It's hard to judge from your images. geode or seam agate / chalcedony - all the same material. the word banded agate, bands form inward to center, as dos seam agate from two directions. Both have the same formation. You may need micro eyes to see. where you find the iris. in the field of bands. Good example is Holley blue agate seam or geode, Holley blue jelly cant see the bands, Though you know their there. Because its a Sedimentation formation.... Add a little dirty iron and you can see each of the formation rings inward. Pillow basalt most often, is the home port of all agate chalcedony formation. Then the grind too end up down flow to agatize Petrified wood & fossils , Chert - Jasper's.
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gemfeller
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Post by gemfeller on Jan 15, 2019 22:40:06 GMT -5
Lapidaryrough / Jack Cole , I'm not wedded to that definition. That's why I qualified it by attributing it to mineralogists. It's also the definition used by many gemologists. But I agree there's a very fine line between the two. For instance here's a Blue Lace Agate cab showing banding, but if cut another way it will be pure blue without bands: This material from Malawi is sold as Blue Chalcedony. But when viewed from the edge it shows banding: But from another angle it shows a pure blue botryoidal surface: Go figure!
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rockhoundoz
starting to spend too much on rocks
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Post by rockhoundoz on Jan 15, 2019 23:39:13 GMT -5
Thanks to yall for the thoughts!
I know my cell phone pics aren't the best, but those colors from the two pictures I posted are actually pretty true to the colors my eyes see on the actual specimens in that lighting.
Definitely the background changes the color. The natural bamboo sets a nice contrast to highlight the blue color. The white background creates the same color/lack of color for the stones as when I shine an led through them, if that makes sense, which explains my conclusion about 'transmitted light'.
I'm close enough to chalcedony deposits that it seems like a very reasonable conclusion. I guess ultimately, viewing the crystalline structure under magnification would be conclusive for chalcedony vs 'rock quartz'?
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rockhoundoz
starting to spend too much on rocks
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Post by rockhoundoz on Jan 16, 2019 0:57:34 GMT -5
Looks like chalcedony to me. Mineralogists tell us the difference between chalcedony and agate is that agate is banded and chal. is not. As for color, the only real test is in natural light with a neutral background. It's hard to judge from your images. Also, what's the best/easy best 😅 neutral background for pictures of this kind ?
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fuss
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Post by fuss on Jan 16, 2019 9:55:59 GMT -5
Looks like chalcedony to me. Mineralogists tell us the difference between chalcedony and agate is that agate is banded and chal. is not. As for color, the only real test is in natural light with a neutral background. It's hard to judge from your images. Also, what's the best/easy best 😅 neutral background for pictures of this kind ? Google "Rayleigh effect in quartz" and then try a few tests with different light.
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dreamrocks
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Post by dreamrocks on Jan 16, 2019 10:09:17 GMT -5
Here's a stone I've been collecting in gravel from a location in east coast piedmont country. My immediate thought was quartz, either 'blue quartz', 'blue chalcedony' or even faded amethyst. Then I noticed that the color of the stone changes under different lighting &considered zoisite or iolite. Out of all the stones I mentioned, zoisite is the only one known to occur in this region via mindat. Here are two pictures. I suppose that having the white paper towel under the stones creates 'transmitted light'. Since blue chalcedony is known to be yellow/tan/colorless while having 'transmitted light', but appears blue when lighted from above, I suppose that's evidence for blue chalc. Hardness is around 7 & streak is white to colorless. Any opinions? Again, the lighting in the 2 pics are identical (indirect sunlight), the only difference is white background vs bamboo background. Thanks! That's some juicy looking stuff. I check for morganite and aqua using white paper when the stone is very pale. The white paper is a easy test with no gem tools. Next thing I notice is a definite color change but with a twist the bamboo there are at least 3 stones looking botroidal the others a agate look. Pinkish purple showing if you look close at the white paper plus the stones are turned upside down from bamboo picture Personaly instead of guessing what it is contact John Betts fine minerals or GIA and see what it's gonna take to have them ID it. If a color change stone due to lighting just couple bucks more with there ID if rare who knows. Just saying that's what I would do a trade the smallest stone for ID in writing!
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Deleted
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Post by Deleted on Jan 16, 2019 13:16:44 GMT -5
Also, what's the best/easy best 😅 neutral background for pictures of this kind ? If your camera has a "White Balance" feature, use that. For those that don't (or when shooting pics that don't contain any pure white), I find a neutral light-gray gives the best approximation of the in-hand appearance. Photography stores (online and off) sell sheets of paper with gradient backdrop patterns from medium gray to white that do a decent job. If you have a piece of brushed stainless steel lying around, that also works in bright light. I shoot under natural sunlight, and used to do a better job getting correct colors, but have gotten less patient over the years.
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gemfeller
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Post by gemfeller on Jan 16, 2019 13:27:00 GMT -5
Looks like chalcedony to me. Mineralogists tell us the difference between chalcedony and agate is that agate is banded and chal. is not. As for color, the only real test is in natural light with a neutral background. It's hard to judge from your images. Also, what's the best/easy best 😅 neutral background for pictures of this kind ? The colors usually considered "neutral" are white, gray, beige and black. For gem photography purposes you just have to avoid backgrounds that influence the hue of the stone being photographed by reflection. For instance, I photograph a lot of transparent faceted gems. I couldn't figure out why I was picking up unnatural tints in my pictures until I realized the color of the shirt I was wearing or other main colors in the room were being reflected from the subject stones. The test of color change in gems is the color in daylight vs. the color in incandescent (tungsten) light. A color change gem like Alexandrite will show a complete hue change, from blue/green in daylight to purplish-red in incandescent light. Stones that exhibit so-called "color-shift" may show variations of a single hue in different lighting conditions. For instance, tourmaline from some sources appears strongly pink-red in daylight but shifts to orangy-pink under tungsten. ETA: R2D's comment about correcting white balance is very important. Each camera has a different system for those adjustments, check your manual.
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rockhoundoz
starting to spend too much on rocks
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Post by rockhoundoz on Jan 16, 2019 18:45:53 GMT -5
Also, what's the best/easy best 😅 neutral background for pictures of this kind ? Google "Rayleigh effect in quartz" and then try a few tests with different light. Thanks, good to know! So 'blue quartz' looks blue due to the rayleigh scattering effect, which is a result of light being 'scattered' by tiny ilmenite inclusions in the quartz. Coincidentally, ilmenite is listed to occur where these pieces were found. Blue chalcedony, looks blue due to the tyndall effect, which occurs based on the physical shape of chalcedony fibers in the stone.
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rockhoundoz
starting to spend too much on rocks
Member since March 2017
Posts: 131
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Post by rockhoundoz on Jan 16, 2019 18:54:21 GMT -5
Here's a stone I've been collecting in gravel from a location in east coast piedmont country. My immediate thought was quartz, either 'blue quartz', 'blue chalcedony' or even faded amethyst. Then I noticed that the color of the stone changes under different lighting &considered zoisite or iolite. Out of all the stones I mentioned, zoisite is the only one known to occur in this region via mindat. Here are two pictures. I suppose that having the white paper towel under the stones creates 'transmitted light'. Since blue chalcedony is known to be yellow/tan/colorless while having 'transmitted light', but appears blue when lighted from above, I suppose that's evidence for blue chalc. Hardness is around 7 & streak is white to colorless. Any opinions? Again, the lighting in the 2 pics are identical (indirect sunlight), the only difference is white background vs bamboo background. Thanks! That's some juicy looking stuff. I check for morganite and aqua using white paper when the stone is very pale. The white paper is a easy test with no gem tools. Next thing I notice is a definite color change but with a twist the bamboo there are at least 3 stones looking botroidal the others a agate look. Pinkish purple showing if you look close at the white paper plus the stones are turned upside down from bamboo picture Personaly instead of guessing what it is contact John Betts fine minerals or GIA and see what it's gonna take to have them ID it. If a color change stone due to lighting just couple bucks more with there ID if rare who knows. Just saying that's what I would do a trade the smallest stone for ID in writing! That's an interesting observation about the botyroidal shapes, dreamrocks, I didn't notice until you pointed that out! After closer observation, there are no botyroidal shapes on the surface, but there do seem to be internal features which repeatedly cause a reflection of sorts that appear globular. On that subject, I found several chunks of this stuff which have ribbon-like inclusions of an translucent whitish/grayish mineral, which isn't 'regular' enough in appearance that I'd call it agate, however, I assumed that those inclusions at least were chalcedony. Also, absolutely, I plan to take these in to a gemologist in person to have a better answer. May also attempt some microscopy in the meantime : ).
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NRG
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Post by NRG on Jan 16, 2019 19:07:59 GMT -5
Are these two images taken in identical light and the only difference is the white paper?
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rockhoundoz
starting to spend too much on rocks
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Post by rockhoundoz on Jan 16, 2019 19:16:47 GMT -5
Thank you for the photo & lighting advice, rocks2dust & gemfeller! Unfortunately, I didn't get a chance to attempt new pics today until after sunset, but gave it a shot anyways with the low light, using brushed steel. The colors are expectedly muted, looking much less colorful than full lighting, but I hope to try tomorrow midday. Those pictures above were all taken with the camera pointing away from the (setting) sun. The following picture was taken facing the sun, aka transmitted light. You can also see in all the pictures, how orienting the stones differently creates a certain amount of the yellow/pinkish color. Next, a couple other specimens from the same gravels. Until finding this next piece, never would've expected to find agate here. It's possible that this little guy travelled some 20/30 miles across drainages, from known ultramafic agate occurrences. & then some really nice clear gemmy smoky quartz (with poor lighting), & also noticed in this pic, that chunk of the material under duscussion, the orange light reflected/refracted(?) onto the white paper could indicate Rayleigh scattering: & one final pic, some nice serpentinite from those ultramafic deposits 20 miles away : )
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rockhoundoz
starting to spend too much on rocks
Member since March 2017
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Post by rockhoundoz on Jan 16, 2019 19:19:25 GMT -5
Are these two images taken in identical light and the only difference is the white paper? Yes, the only things different were the white paper, & also, unintentionally, the stones were rotated in a different direction. But they still have that color change in those conditions regardless of orientation of stones.
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goatgrinder
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Post by goatgrinder on Jan 16, 2019 20:54:42 GMT -5
What an interesting collection of hypothesis, opinion and science. I cannot wait to read the next chapter.
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Post by fernwood on Jan 17, 2019 4:03:04 GMT -5
Thanks for the updates. The different angles, light and backgrounds sure make a difference.
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