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Post by knave on Sept 1, 2020 13:10:50 GMT -5
The term “Hertzian cone fracture” was new to me. Thanks Bob!
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rockstock
spending too much on rocks
Member since April 2019
Posts: 472
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Post by rockstock on Sept 1, 2020 13:27:18 GMT -5
That Brazilian looks awesome. Haven't seen anything like it.
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Post by rockpickerforever on Sept 1, 2020 13:36:49 GMT -5
If someone can mention what setting to change to make sure photos are not this gigantic, I will fix. Great job on that Brazilian! You, sir, have amazing patience. It paid off with that amazing stone. When I edit my photos before uploading, I have found that a width of 600 pixels is a good compromise. Big enough to see, not too big for folks viewing on cell phones. You can also edit the size once in Cloudinary, but if you do it before uploading, your photo will will take up less of your allotted space. Nice work!
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Post by miket on Sept 1, 2020 13:37:22 GMT -5
That's beautiful, I'm glad you're posting pictures now and can't wait to see more!
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Post by Bob on Sept 1, 2020 14:07:10 GMT -5
I will continue to post some more, and spread them out over time. There are a few that I think will be some nice surprises. My largest, not yet done, probably weighs 10x that one. FYI, many rockhounds know that when they see a certain pattern on this or that rock scattered among lousy rocks in a stream, that that pattern indicates a high Mohs and possibly an agate. That surface pattern is the Hertzian cones caused by impact with other rocks. But, if the rock is pretty round, it's very difficult to fracture very far w/o tremendous forces, and thus the Hertzian cone pattern on the surface only. One day I started looking closely at this pattern with a 10x hand lens and starting coming to understand it. I'm guessing that in agates with their fibrous cryptocrystalline structure, that these Hertzian cones can just barely happen on the surface and therefore never "lead" into an open conchoidal fracture. I'm not a knapper and don't have any interest in it. One of my friends does this and was over at my house looking at my chert last year. He picked up a choice piece, asked me if he could work it a bit, and whacked with a hammerstone he had. I was in shock when I saw it cleave almost 90% to the direction of his impact swing and a large smooth piece fall off the BOTTOM and into his hand. That got me interested in coming to understand a bit about how these Hertzian cones propagate and allowed the aboriginal peoples to work this material into cutting edges so effectively. For anyone who wants to come to a bit of an understanding of the amazing nature of this method of cleaving rocks and the resulting surfaces, check out the slides from this Univ of Wash course anthropology.washington.edu/courses/2019/spring/archy/483/aI find these slides and associated material just incredible and learned a lot that has helped me spot things in the field. Now I understand why what happened happened when I tried to cleave my first large piece of obsidian in half. The results were very unexpected by me then. I had a piece about twice the size of a pack of cigarettes in all directions. I whacked it in the middle with a brick chisel. It split not at all the way I hoped, but split in half mostly in the thin dimension! So I ended up with exactly what I didn't want to happen--two thin flat pieces. Now I saw obsidian.
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Post by Mel on Sept 1, 2020 18:09:27 GMT -5
Wow!!! Share more, please!
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Post by Bob on Sept 2, 2020 10:41:48 GMT -5
One thing I should have mentioned. The back side of that agate is virtually a perfect sphere you can probably tell. In fact, it's so blemish free that that side look like manmade glass. But, the middle of the broken side, the inside of the agate, is actually got a concave dented part in the middle. It took forever to grind that out in from the rough. At one point I got that indention down to only maybe 1/8" deep, and figured it would be gone in a couple of months. Nope, it took nearly another year.
That experience as well as some others convinced me that if I could grind out some things, I could save some money on grit, so I bought a 8" wet arbor. I have not got around to setting it up yet, but sure have a lot of rocks backed up for it.
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Post by Bob on Sept 2, 2020 10:52:47 GMT -5
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Post by knave on Sept 2, 2020 12:03:46 GMT -5
Makes me want to hold it in my hand, such a cool rock.
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Post by Bob on Sept 3, 2020 17:28:23 GMT -5
I would love to know how much it cost me in terms of grit, but figuring that out would be next to impossible because it wasn't the only rock being tumbled.
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Post by Bob on Sept 3, 2020 17:39:31 GMT -5
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Post by Bob on Sept 3, 2020 17:45:01 GMT -5
I have dreams about maybe selling some of my monster tumbles someday. So I designed this tracking sheet. It's a lot of work to do, but I have started it on a few big ones. It was designed to cope with the situation where a single big rocks is pulled out processing, might sit on the shelf for a few days or weeks or months, then go back in. So the Total Days: field looks like this in use. 7+7, 7+7+7+7, 7+7 = 56. That rocks ran for 8 x 7 days weeks, but was pulled out of processing twice as represented by the commas. And I don't care how long it was out of processing and sat on the shelf. It's only how long the rock truly was tumbling that I want to measure. I should post sometime one of these sheets that is in actual use and partially completed. Of course, they get real dirty real fast laying around in my tumbling area and getting handled by my dirty wet hands.
Also, I thought it might be fun if you bought one of my big rocks, to have documentation that the thing took 595 days or whatever it was, and started out that big and ended up this big.
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Post by knave on Sept 3, 2020 19:33:34 GMT -5
At first blush, logging all the deets seems a touch boring and nerdy, but the more I do this, the more important it becomes to track your inputs and results.
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Post by Bob on Sept 3, 2020 22:53:51 GMT -5
That's only for my really big rocks, orange size or larger are big to me.
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Post by Jugglerguy on Sept 4, 2020 18:51:16 GMT -5
One thing I should have mentioned. The back side of that agate is virtually a perfect sphere you can probably tell. In fact, it's so blemish free that that side look like manmade glass. But, the middle of the broken side, the inside of the agate, is actually got a concave dented part in the middle. It took forever to grind that out in from the rough. At one point I got that indention down to only maybe 1/8" deep, and figured it would be gone in a couple of months. Nope, it took nearly another year. That experience as well as some others convinced me that if I could grind out some things, I could save some money on grit, so I bought a 8" wet arbor. I have not got around to setting it up yet, but sure have a lot of rocks backed up for it. I was about to ask why you didn't grind some spots out. That should save a ton of time. After the coarse stage, do you run the rock by itself with media? Do you use ceramic or plastic media? That rock is amazing.
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Post by Bob on Sept 4, 2020 20:19:02 GMT -5
I didn't give it any special treatment at all. It was just another rock in the barrel.
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doublet83
starting to spend too much on rocks
Member since March 2016
Posts: 118
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Post by doublet83 on Sept 5, 2020 8:12:41 GMT -5
Very cool, thanks for sharing your story here. I've tried tumbling big rocks but I've learned I really hate how long it takes and how it bruises the other rocks, so these days I really try to keep it under 8 ounces. Tumbling a piece for years is really an exercise in patience.
I've seen Australians bring huge 20 pound tumbled mookaite to the rock shows selling for a few hundred dollars, although they are not tumbled to the point where they are well rounded, just generally smoothed. I'm not sure you can ever really make money selling tumbles because you're competing with huge operations like the ones in Africa where they use highly efficient tumblers with capacity that is over a ton, and labor that is really cheap.
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jamesp
Cave Dweller
Member since October 2012
Posts: 36,154
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Post by jamesp on Sept 5, 2020 8:40:34 GMT -5
I didn't give it any special treatment at all. It was just another rock in the barrel.
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RWA3006
Cave Dweller
Member since March 2009
Posts: 4,124
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Post by RWA3006 on Sept 5, 2020 9:45:03 GMT -5
One thing I should have mentioned. The back side of that agate is virtually a perfect sphere you can probably tell. In fact, it's so blemish free that that side look like manmade glass. But, the middle of the broken side, the inside of the agate, is actually got a concave dented part in the middle. It took forever to grind that out in from the rough. At one point I got that indention down to only maybe 1/8" deep, and figured it would be gone in a couple of months. Nope, it took nearly another year. That experience as well as some others convinced me that if I could grind out some things, I could save some money on grit, so I bought a 8" wet arbor. I have not got around to setting it up yet, but sure have a lot of rocks backed up for it. I was about to ask why you didn't grind some spots out. That should save a ton of time. After the coarse stage, do you run the rock by itself with media? Do you use ceramic or plastic media? That rock is amazing. Interesting thread. I can't but help think a super grinder such as what James put on a tile saw would smooth out some of the worst and especially the concave areas of the rock and thus save maybe months of tumbling?
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kyoti
has rocks in the head
Member since June 2020
Posts: 542
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Post by kyoti on Sept 5, 2020 11:21:57 GMT -5
Thanks for sharing all your experiences Bob. That Brazilian stone is a beauty! Reading your post is making me want to try tumbling one of my bigger nodules. I agree with the others about using a saw or grinder to help shape the stones. I only have a 3 lb barrel but I've taken to using my tile saw to cut off all the spiky and badly shaped bits. It saves a tremendous amount of time in the rough. It's also cut down on a lot of the spalls and cracking I had previously.
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