High Bruneau desert Idaho, appears a small percentage with the crystal like center, most with stone center, and, some seem to encase a smaller stone sphere
Reminds me as a kid using a screwdriver and hammer to dislodge garnets from a large rock and collecting them in a Mason jar. Thanks for the memory reminder.
Don.....
This outcrop intrigues the hell out of me. I can’t believe there is nothing in the geologic literature of the area that mentions these red sandstone nodules in a black matrix.
I asked the Idaho Geological Survey about this outcrop -here is their response:
It looks like you have some well-developed spherulites that formed in what is almost certainly a rhyolite. These form when the original glass recrystallizes as feldspar and quartz. When hollow, the term lithophysae is used, and that is how you get thunder eggs, the Spencer opal deposits, and geodes. Those with the crystals in the center are geodes. Hot fluids percolating through the rhyolite after the spherulites form carry the silica needed to form opal, agate, and quartz crystals.
These spherulites are probably from one of the many rhyolitic ash-flow tuffs in the region that formed in massive eruptions as a result of the passage of the Yellowstone hot spot. Rhyolite tuffs form when hot ash lands and welds back together. If that happens quickly enough, glass forms. It is that glass that will sometimes recrystallize.