I was rip-rapping my banks and found this in one of the limestone chunks.
Looks like a bone of some kind.
I found more in another limestone rock but have not got it out yet. Took me all day to get this one chipped out with a hammer and chisel. Will post that one when I get it out. It is shaped differently.
I see some have been looking at these but not any ideas yet. Someone told me that it may be a geode, however I don't think so. The other formation in the other rock is really more bone shaped than this one. But I don't know. Hope someone out there does.
This message was edited Aug 7, 2007 5:22 PM
Jmorth, I don't think so either. I looks like bone fragments of some large animal to me.
Without being able to see them in hand, the samples appear to be concretions, with poorly-crystallized centers, hence, geodes, albeit not the well-crystallized sorts that collectors prize. What is the crystallization on the inside - calcite or quartz?
Well, calcite is the mineral that makes up limestone. The standard test for limestone and calcite is to put a drop of dilute hydrochloric acid (10% HCl) on it - a fizzing reaction indicates calcite; the fizzing stops when the acid is spent; to stop the reaction, just wash the acid off with water. So you could test both the outer white part of your concretions, which you say is limestone (and it does look like chalky limestone), as well as the inner crystalline-looking part.
Concretions/geodes of chert are also common in limestone (and chert is just a form of quartz). A test of hardness would also distinguish between quartz and calcite. Calcite can be scratched with a pocket knife blade, while quartz can't be.
This message was edited Nov 20, 2007 9:25 PM
Geodes form from mineralization growing inward...concretions grow from accumulations around a nucleus.
Tried to scratch with knife. Nope cannot scratch. Left marks on the stone from the knife.
When I was chipping this out of the limestone a small slice came off. If you would like me to I could mail it to you.
So, it sounds like it may be a nodule of silica (e.g. quartz, chert), which, as I mentioned, occur commonly in limestone strata. That might be about all I could say without seeing it and testing it. Thanks anyway, but there's no need to send me a sample of it. If you live near a university, you may be able to get it identified more conclusively by the geology department there.
Jmorth, your point about the origins of geodes and concretions is a good general rule, and I was being fast and loose with the terms... Also, I was not too sure of how strictly rock collectors follow those rules (or how they would test them in cases where it is not clear? For example, thin section examination can unravel complex solution and crystallization histories, but I doubt many collectors would get in to this...though I could be wrong.)
Just wondering though - what do you see in the photos that would convince you that it's definitely not a geode?