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I am looking for information on drusy quartz of potosi. I have several large rocks, would like info.?
We picked these up when I was a kid and put in flower beds. At the time they were just pretty. I would like to know the geological formation, etc. Also if there is any value. Some are over a foot in diameter.
- 1 decade agoFavourite answer
What you probably have is also known as Potosi Agate, Missouri Lace Agate, Missouri Agate, and possibly several other names. It was formed in place in the Potosi Formation of Late Cambrian Age from acid groundwater perculating through the rock, which also produced the red, sticky clay residue called "terra rosa" that this drusy quartz is found in.
The normal use and highest value for such rocks are exactly the use you gave them originally. A large or unusual example may bring a few dollars at a flea market, though outside of the area where they commonly occur I would expect novelty might increase that price slightly. I would be very surprised if the value of a given specimen would be nearly as much as normal shipping charges, though. Sometimes the banded agate that forms the base of the druse is thick enough to be used as a gemstone of low to moderate value, similar to other common agates. I've seen some fairly attractive folk jewelry at craft festivals around the Ozarks that I strongly suspect were made from Potosi Agate.