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Post by rockjunquie on Feb 28, 2023 18:12:00 GMT -5
Is it to increase the depth of the color in the stone? Maybe this is a stupid question, but I have always wondered.
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Post by Pat on Feb 28, 2023 18:40:27 GMT -5
Wouldn’t that be called a two-sided cab?
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Post by rockjunquie on Feb 28, 2023 18:41:49 GMT -5
Wouldn’t that be called a two-sided cab? I know, right? It wouldn't fit the definition for the cab contest.  |
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Post by hummingbirdstones on Feb 28, 2023 18:48:24 GMT -5
The Aussies almost always cut opal cabs that way. Increases the carat weight and the price.
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Post by rockjunquie on Feb 28, 2023 18:52:29 GMT -5
The Aussies almost always cut opal cabs that way. Increases the carat weight and the price. I notice a lot from India are done that way, too. A lot of labs and moonstones have slightly convex backs. Annoys me. LOL! |
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Post by hummingbirdstones on Feb 28, 2023 19:01:42 GMT -5
The Aussies almost always cut opal cabs that way. Increases the carat weight and the price. I notice a lot from India are done that way, too. A lot of labs and moonstones have slightly convex backs. Annoys me. LOL! It annoys me, too.  |
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herb
spending too much on rocks
Member since November 2011
Posts: 475
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Post by herb on Mar 1, 2023 10:07:16 GMT -5
If the cab is hand done, maybe it's convexed because it is faster / easier to polish the back that way? With a flat back it can be more time consuming to work out dull spots in the middle, especially with bigger cabs
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Post by rockjunquie on Mar 1, 2023 10:27:28 GMT -5
If the cab is hand done, maybe it's convexed because it is faster / easier to polish the back that way? With a flat back it can be more time consuming to work out dull spots in the middle, especially with bigger cabs I hadn't thought of that! Could be. |
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Post by 1dave on Mar 1, 2023 10:29:38 GMT -5
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gunsil
spending too much on rocks
Member since January 2023
Posts: 345
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Post by gunsil on Mar 1, 2023 12:20:07 GMT -5
I see that on a lot of moonstone and amethyst cut in India or Thailand. I think it is just the way they cut and polish them on automated machines. They might just shape the stones and tumble the rest of the way, that would give the convex back. I bezel set a lot of amethyst and if it has a curved bottom I use fine silver for the back and shape it to match the curvature of the back. If I find an opaque stone with a convex back I just whack the back on my rotary flat lap, just takes a minute. Most of my stones I cut and polish myself but amethyst and good moonstone rough is hard to find so I buy pre made cabs. I don't think it increases depth of color, I think it is just the result of manufacturing means. Doesn't seem like it would be a problem for wrappers but since I don't do that I really don't know.
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Post by Rockoonz on Mar 1, 2023 13:00:10 GMT -5
I have made a few double dome cabs, but only for drilled focal beads so far. Good if there's a thick slab with both sides nice looking. It used to be a common practice and I've seen open settings and wire wraps designed for them.
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Post by rockjunquie on Mar 1, 2023 13:34:20 GMT -5
I have made a few double dome cabs, but only for drilled focal beads so far. Good if there's a thick slab with both sides nice looking. It used to be a common practice and I've seen open settings and wire wraps designed for them. I'm not referring to a double domed cab, just the ones with a slightly convex back. (Not meant to be seen.) I agree a double dome cab could be cool- esp for a 2 sided groove wrap. |
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Post by rockjunquie on Mar 1, 2023 13:35:47 GMT -5
I see that on a lot of moonstone and amethyst cut in India or Thailand. I think it is just the way they cut and polish them on automated machines. They might just shape the stones and tumble the rest of the way, that would give the convex back. I bezel set a lot of amethyst and if it has a curved bottom I use fine silver for the back and shape it to match the curvature of the back. If I find an opaque stone with a convex back I just whack the back on my rotary flat lap, just takes a minute. Most of my stones I cut and polish myself but amethyst and good moonstone rough is hard to find so I buy pre made cabs. I don't think it increases depth of color, I think it is just the result of manufacturing means. Doesn't seem like it would be a problem for wrappers but since I don't do that I really don't know. I can't speak for everyone, but as a wrapper, I find those convex backs to be problematic and they rock- which I don't like. |
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Post by Starguy on Mar 1, 2023 13:45:26 GMT -5
I’ve seen a lot of star sapphires cut with rounded backs. I always assumed it was to increase the carat weight of the stones. The stars show just as good with a flat back.
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Post by Rockoonz on Mar 1, 2023 13:51:04 GMT -5
I have made a few double dome cabs, but only for drilled focal beads so far. Good if there's a thick slab with both sides nice looking. It used to be a common practice and I've seen open settings and wire wraps designed for them. I'm not referring to a double domed cab, just the ones with a slightly convex back. (Not meant to be seen.) I agree a double dome cab could be cool- esp for a 2 sided groove wrap. Groovy. I wonder if a lot of the commercial cabs like that aren't quickly shaped and rotary or vibe finished, flats and concaves are the last thing to polish in a tumbler so a little convex would speed up the process. When we were getting started we bought a lot of cheap, large imported "cabs" when they looked big enough to cut down into something nice and cost less than a slab. |
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gemfeller
Cave Dweller 
Member since June 2011
Posts: 4,060 
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Post by gemfeller on Mar 1, 2023 16:52:41 GMT -5
The Aussies almost always cut opal cabs that way. Increases the carat weight and the price. And it's much easier to polish a convex surface than a flat. |
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gemfeller
Cave Dweller
Member since June 2011
Posts: 4,060
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Post by gemfeller on Mar 1, 2023 16:57:25 GMT -5
I’ve seen a lot of star sapphires cut with rounded backs. I always assumed it was to increase the carat weight of the stones. The stars show just as good with a flat back. It's traditional in India and other Far East cutting centers. I have a parcel of India-cut cat's-eye sillimanites and some have so much "belly" it nearly doubles their weight. |
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