jamesp
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Post by jamesp on Apr 23, 2013 18:43:24 GMT -5
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Deleted
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Post by Deleted on Apr 24, 2013 13:19:42 GMT -5
That is really interesting. You really continued your homework after we discussed this. As stone cutters we get a feel for the hardness of a stone. We know your corral is super duper hard because we handle it and cut it and learn. We also can say, this jasper is harder than xyz.
The question I have is how do they say 7.0-7.1.....7.4 with such precision? I would love to know more about this. Is it just experience or is there some tool or method?
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Post by deb193redux on Apr 24, 2013 16:47:40 GMT -5
I don't for a minute believe "rikoo", which is a seller has conducted very precise scientific hardness tests.
... and hardness can vary within a material. I think they just made those number up.
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jamesp
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Post by jamesp on Apr 24, 2013 17:41:40 GMT -5
I respect the 16 inch spheres they make(or sell).Someone is a very experienced lapidary person.And i have worked with bloodstone and coral.And i have old timer friends who have worked with Florida coral for years.And i have tumbled 100's of pounds of mixed Rio Grande agates with coral and i will assure you the coral is dang hard.And indian bloodstone tumbles like coral.Two others that i would guess to be harder are Montana and Brazillian.I find it interesting that silicified wood often breaks along the grain.Palm is hard to cut petrified or not.And coral is hard before it silicifies and for sure after. My experience is the slower the material tumbles off edges the harder it is.And these are the materials that have the highest shine in their polish. Rockwell(metals) and durometer(rubbers) are easy to test;i would guess moh is too.I looked at dozens of materials they made spheres out of and their posted mohs looked accurate.Maybe from a book.But i don't think they made them up.Any thoughts?
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Post by deb193redux on Apr 24, 2013 20:11:07 GMT -5
I'm not saying the did not have some reference materials, but they are not a scientific source, and they do not cite their source. and I find differences in hardness within materials, within Brazilian, within corals. Their statements are thus very general. Maybe bloodstone and fancy jasper is pretty consistent, but jaspers vary a lot. They are quoting MOHS to the 1/10 unit, and I have never seen information that precise in lapidary books. I still think they had a "sense" of things and ad libbed a bit. In fact I read that MOHS is only precise to the half unit (e.g., 6.5). Technically values like 7.3 do not exist on the scale. geology.about.com/od/scales/a/mohsscale.htmprior "hardness" of coral is immaterial because the mineral is completely replaced by silica. It is not additive. Besides, coral is made of calcium carbonate, which is a fairly soft mineral.
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Post by Deleted on Apr 24, 2013 23:59:56 GMT -5
Thanks Daniel. That is the kinda knowledge I was asking about. My guess is this. For some reason fossil coral is harder than other agates, or even quartz itself. I've now cut both and can say that with some surety. If quartz is a 7 and the coral is harder, I'll go with 7.5. I have another question in this line of thought. If Mohs is supposed to be in 0.5 increments and bloodstone is not as hard as fossil coral but is harder than quartz, what should we do?? Hardness is not quantum having levels but indeed will be continuous from one "step" to another. Maybe the solution is the cutter's opinion is what gets reported. "I cut these three stones and I will say that blood stone is sofer than fossil coral but harder than quartz." Had they worded it that way, we would not be learning and chatting. Someone send me about 30# of that bloodstone and I'll see if I can answer that question better!!! Are you kidding me?
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jamesp
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Post by jamesp on Apr 25, 2013 3:07:15 GMT -5
Mohs scale is crap.After study i see why.It is very crude.And even worse,it is way non linear.Going from a 9 for ruby to a 10 for diamond is misleading-cause a diamond should be like 100-it way harder than ruby.And calcite is probably 10 times harder than soapstone. My interest was the relative hardness.And i am impressed with their posted hardnesses.At least they stuck it out there for reference.And jasper does vary-rikoo has it posted from moh 5 to moh 7.5(red snake skin?).And i don't really know much about their(some chinese) jaspers,but i now US ones vary.That bloodstone and the coral i tinker with is a repetitive material and always hard.Montana and brazillian too. The tumbler kit i got 40 years ago came with bloodstone and it was the last to round.That stuff was hard then and still hard today. So i was also trying to figure out why coral is harder.It is a chemical compound that must consistantly have some chemical in it that makes it harder?Molecule arrangement? And lots of complaints about coral wearing out the 100 wheel on the cab machine-that is a big problem. Great info.Learned a lot
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Post by Deleted on Apr 25, 2013 9:52:12 GMT -5
Well, I got to thinking about this question. Perhaps an analogous material is gorilla glass. Gorilla glass is glass, sure. But it is WAY harder and WAY tougher. Here is how it is made. www.howstuffworks.com/everyday-tech/gorilla-glass.htmThey take the specially made glass, then dip it in a molten pot of potassium salt. At these temperatures the potassium replaces the sodiums in the glass. When the change has been made and the glass cooled and cleaned it is now way tougher. This is because the potassium atoms are bigger in size than the sodium atoms. The larger size of the atoms locks the matrix together in a very tough fashion. Maybe your coral is similar.
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Post by deb193redux on Apr 25, 2013 10:02:38 GMT -5
There are absolute hardness scales where differences have real meaning, unlike MOHS. But ordinary folks cannot make these determinations as easily as they can determine relative MOHS.
I still say fossil coral is a broad category. Technically it includes petoskey stone, which is not even agate. I am also not sure how comparable the Indonesian coral on the market today is to the coral heads in West Virginia to the coral materials in Florida.
What the lapidarist perceives as hardness is generally a combination of hardness, density, strength, and crystal properties. All of these are different things. The lapidarist just sees it takes longer to work and is more difficult to break.
Assuming agatized indoneasian coral is harder than other agates, we could speculate about why that might be. Biogenic source of silica? Marine formation? Temperature of formation? - it would be something about the rate of silica precipitation from fluid and crystal formation. Generally I think smaller finer crystals will be stronger, but I also think the organization of the crystal patterns (not just size) will come into play. After all, organization of the molecules is the whole theory of tempering metals for strength.
Possibly agates that are replacing structures (like coral) are stronger than agate that forms in larger voids.
This is assuming the structure is completely replaced. pet wood sometimes separates on the grain because replacement is incomplete - not because it is softer. I sometimes find coral that is not fully silicified, and it is low density, porous, lapidary junk. It would be interesting to get an absolute hardness test on a fully agatized wood with grain (replacement) versus a limb cast (void). I think the limb cast might be softer, but this is just a guess.
I think "source" and "rate of transport" of the silica are more productive areas to look. I think biogenic sourced silica is often transported and precipitated at lower temperatures that silica from volcanic ash. This might make for finer more organized crystals deposited in a series of laminations that could result in a harder material.
These are just hypotheses. But they do follow logical use of what is known about different physical properties, and about differences in how various agates form.
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jamesp
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Post by jamesp on Apr 25, 2013 10:09:23 GMT -5
That makes sense.Well it's hard allright.Too hard.I got to dig stuff like this up or sleeping disease takes over. I was trying to figure out if wood silicifies vs palm vs coral vs oyster vs dino bone-which is harder and why.Molecular alignment,type of chemicals,etc It is complicated and variable but the coral was so consistent it made me curious. Thanks for the input Daniel and Scott.
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jamesp
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Post by jamesp on Apr 27, 2013 6:41:27 GMT -5
Sorry Daniel,I did not see your last post above.You posted while i was typing. Your thoughts on silicification are very accurate to me.And it is real interesting to know how silicified materials are.And their behavior afterward.I have a lot of experience with a few materials.My local coral is spread out over upper Florida/S.Georgia and varies a lot.Many large chunks are silicified on the edge and not in the center.Most small less than 3 inches are silicified all the way thru.Some 200 pounders are silicified all the way thru and ting like a bell.I know that coral from the southern coral zone tumbles quicker than the Georgia stuff. The theory of formation is cool to think about.Volcanic vs fossil,cast vs replacement.Like a book i got on L. Superior agates is highly technical and it looks like scientists have got those agates figured out pretty dang well.I wish those scientists would pay Florida a visit and solve my little coral mysteries. The coral in the Withlacoochee River.It is hard,it rings,it is large,it is large and often fracture free,when silicified it is consistent.But it is always hard.And stubborn.And complaints get filed against it by cabbers.For wearing out their 100 wheel(i think,i am not a cabber).And i make my own tumblers that run crazy fast so i can tumble the irritatingly stubborn material.Often having to pregrind it on my own grinder to assist tumbler. So i am in my own little world tinkering with this local coral.Like heat treated carbon steel,it don't grind fast et al.Funny,i heat treat it and it is a lot softer and chips way easier-but still agate like in it's properties. Then i found another location in Florida with a coral that does not ring.And does not chip like glass.It is not only hard,but it is tough.Like a file will snap,but is very hard...This stuff is both hard and -tough for a lack of better word-it will not chip like glass or Withlacoochee coral. And rarely large. I have found corals 3-4 feet across that are probably silicified all the way thru.With very few fractures.That ting like a china plate.I am not aware of such large silicifications,not comparing or bragging,just trying to understand what process would create such a killer silicification.And making people aware of what i find down here. It is all interesting and i am a slave to my curiosity.Looking for answers
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Post by deb193redux on Apr 27, 2013 22:41:45 GMT -5
the skeletons of marine critters over eons can provide a lot of silica. when you think about transport into 3ft coral head, I guess it compares to transport into huge agate veins and boulders in volcanic ash. 3ft is a big coral head, but many petrified trees had 3ft diameter trunks. silica has a strong affinity for oxygen, hence Sio2, and is fairly transportable in solution. Lucky for us admirers of micro- and crypto-crystiline quartz.
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jamesp
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Post by jamesp on Apr 28, 2013 6:46:05 GMT -5
The more i work with these agates and silicifications i find fractures bothersome.The bloodstone,coral,Montana and Brazillian is often fracture free.
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