January 8th, 2020 Update:
Master agate and geode cutter and polisher Jeffrey Anderson says what I found was a thunderegg, similar to what I bought from Christopher at the Basin Range Volcanics Geolapidary Museum near Deming, New Mexico. Anderson’s comments:
“That geode is what would be generally referred as “thunderegg” and it’s lined with chalcedony and quartz. The “thundereggs” are associated with any rhyolitic deposits. It is common for chalcedony in thundereggs to fluoresce green under shortwave ultraviolet supposedly due to uranium impurities. 🙂 It is a nice find. :)”
Jeffrey Anderson’s store is called Dwarves Earth Treasures. He will be at Quartzsite this year. Here is his website: http://www.sailorenergy.net/Minerals/MineralMain.html
Original article follows:
Returned to my collecting area with a rock that showed interesting character but little lime/green under SW UV. Best to break it open to see if any more UV would be revealed. While I hoped for decent response, I was not expecting this rhyolite to be a geode.
Last year I collected a similar looking rock from the Armagosa Valley in Nye County, Nevada. Bryan Smalley cut it open and it was just a sad looking mix of brown rhyolite and quartz. This is now my first self-collected geode. I normally would not hunt them since I have no idea where they might be.
I am still looking for uranium occurrences and have had yet to find any. I may, though, have identified sanidine crystals in tuff so that is a good thing. I’ll be back.
Breaking it Open In The Field
Under SW UV, Along With My First Brachiopod (Shell)
I also found my first brachiopod! Photo below:
I may have also discovered a concretion naturally split apart. See below.
Concretions are typically sandstone based. Those might form around a piece of a shell (limestone in nature) or a bit of calcite. My rock fizzes hardly at all, which makes me think it is not sedimentary. Bates and Jackson say concretions can also form around a leaf, bone or fossil and that concretions can derive from “fragmental volcanic rocks.” I’m still reading up on this.
Picture below is my rock and a reference sample I have from Geological Specimen Supply. That rock comes from the Tule Wash in Imperial, County, California.
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