McDermitt Nevada & Transitional Nodules

Many times. Here are the first few.

P. 103
Just as there are degrees across the spectrum
between snowflakes in obsidian to spherulites on up to
defacto mature lithophysae, there is a spectrum of cavity
development ranging from the simplest which is called the
biconoid and the next step above that is that which I have
chosen to call the triconoid. I came up with that term in
1965 in jest to the reluctance of the folks in Templeton,
Calif. to accept the fact that their “Templeton Biconoids”
are thunderegg cores and therefor, Templeton is not the
only place in the world where they are found. The Calif. Division of Mines had told them that they had never
been observed before. We will present the biconoid in Branch Diagram C-2 and the triconoid in Branch

p. 114

I had visited
the fine rock and mineral
collection of the California
Division of Mines when it
was still in the Old Ferry
Building in San Francisco.
In one case were a few
specimens I recognized as
weathered thunderegg cores labeled “biconoids” from Templeton, California, about 20 miles north of San
Luis Obispo. The name tag had a reference number which turned up a paper by Crippen (1948) entitled
Unusual Concretions from Templeton, California. Unfortunately, it was out of print and copies were not
available for sale. However, years later the University of California at Berkeley sent me a copy of that paper.
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Robert Colburn: The Formation of Thundereggs
In that paper I read that shortly before 1948, a local rock hound by the name of C.K. Huff of Paso
Robles, California had submitted some unusually-shaped agate specimens he found in a wheat field by where
he lived, just west of US Highway 101 near Templeton, California. It must be appreciated how strange these
entities must have seemed coming from an area far removed from the famous deposits in Oregon where rock
hounds are familiar with thundereggs, but had never heard that these type of cores were called biconoids.
In the first place, biconoids are rare in most thunderegg deposits and habitual in only a few. So then
it is no wonder, or perhaps quite a wonder when someone drops into a geologist’s office with a hand-full of
strange, highly geometric double-coned objects made of agate, with striations radiating from the center of
both sides out to their edges. Some of them still had lithophysal shells attached to their surfaces,
acknowledged in the paper cited above as some “pyroclastic material” clinging to them. But the conclusions
reached about their origin could only have added more mysteries for descendants to ponder.
The claims that biconoids are found only at Templeton and that they were “casts (pseudomorphs)
of silica jell after calcium carbonate” (Crippen, 1948) were even stranger to me than where they were found.
I knew that these conclusions were in
error the moment I read Figure 5.04
them, but I did not have with me the
“biconoids” I had collected elsewhere,
or the time to debate the issue anyway.
I was far more interested in visiting the
site from which they came, hoping to add
another new thunderegg location to my
collection, regardless of what the cores
were called. I visited the site in 1963,
but it had been picked clean. The land
owner showed me a polished half of the best biconoid I had ever seen.. I asked her if she would sell it and
probably feeling sorry for my lack of luck, sold the one in Figure 5.04 to me for a shamefully low price!

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Chapter Five: The How to Determine Core Structures for Correct Cutting Orientation
The site had been so thoroughly picked over the years that only a few fragments of agate and rhyolite
were found. Everywhere around this field and in the stream channels there were only sedimentary rocks and
alluvium resting on the Monterey Formation. This patch was obviously an erosional remnant of placer gravel
resting on a flat topographic high point. There was no float coming down from the hills to the west, and with
the Salinas River just to the east, the topographic low point at all points in the region, it was unlikely that the
rhyolite fragments had come from the hills to the east. If any source still existed for the biconoids, it would
have to be in the hills to the west. I needed a geologic map and I found the San Luis Obispo Geologic Sheet
in the public library at the county seat, the city of San Luis Obispo.
The map covered the area of interest and it showed a rhyolite flow (misinterpreted as a “rhyolite tuff”
by Taliaferro and Fairbanks [1957]) near the summit of the Santa Lucia Mountain Range. All of the
drainages from this rhyolite flow are now west of the divide which drained west to the Pacific Ocean. The
flow is so close to the divide that it is very likely that it once extended east of the divide, thus being subjected
to erosion that would have supplied the biconoids to the now-isolated placer deposit near Templeton.
I attempted to access the rhyolite flow, but all the land it covered was private, and well-posted: “No
trespassing!” I would have to wait twelve years to gain access, when State Highway 46 was built from just
south of Paso Robles over the hills to the Pacific Ocean. Two thunderegg deposits were exposed in road
cuts just west of the summit. The thundereggs in the large cut on the south side of the highway contained
biconoid cores! Could this flow have been the source of the Templeton biconoids? It would be interesting
to conduct a chemical analysis of the rhyolite on the Templeton biconoids and that on the thundereggs from
the road cuts on Highway 46. I think they would show cogenesis, especially of a high content of iron.
Figure 5.05 is one of the more spectacular biconoids I dug out of the road cut and when I polished
it, it gave off the pungent odor of sulfur dioxide. When I looked at the gray matrix with a magnifying glass,
I saw brassy metallic flecks. The matrix is indurated with marcasite (iron sulfide) which, when exposed to the
oxygen carried by percolating water, is oxidized and hydrated to limonite (Fe2O3(OH))X seen as the
yellowish stains in the quartz biconoid core. It looked like some of the marcasite had rusted and stained the

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Robert Colburn: The Formation of Thundereggs
quartz core. When you look at Figure 5.05
the biconoid in Figure 5.04, it is rusty looking
throughout. Again, I believe that if the matrix of
the Hwy 46 thundereggs and the matrix on the
biconoids were analyzed, there will be a
correspondingly anomalous high percentage of
iron, if not of sulfur. This thunderegg deposit
will be mentioned again because of a peculiar
mineral similarity in all four of the only
thunderegg deposits I know of in the Coast
Range Mountains in California, the high content
of the iron sulfide mineral, marcasite. I have not
found this mineral in any other thunderegg
deposit outside of the Coast Range Province.
For the record, I took some biconoids
I bought in Oregon in 1965 (see Figure 5.01) to
the California Division of Mines and Geology in order for them to update their 1948 interpretation, but as of
1975 the biconoids in their collection still had the same identification label by them, and the old 1948 paper
is still at the University of California at Berkeley Library. The rock hounds in the Templeton area had been
told that Templeton was the only place in the world where biconoids were found, and were not very receptive
to the fact that their biconoids were thunderegg cores which existed in many other deposits in the world!
The Valley View mine operated for three years, producing enough fine agate thundereggs to keep
it struggling until the Lucky Strike really got going. I often think about the tons of biconoids lost in the muddy
tailings. Perhaps some day, when all the best deposits are dug out, another generation will discover just as
many left in the tailings as have been mined, making them worth going through again, with upmost scrutiny.
A thousand miles south of the Valley View mine is another mining claim that failed because of biconic
cores. It is one of the several egg deposits five miles west of Radium Springs, New Mexico, near the center
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Chapter Five: The How to Determine Core Structures for Correct Cutting Orientation
of section 25, T-21-S, R-2-W or near N320 26.830',W1060 59.608' in the Leasburg 7.5 minute quadrangle.
The biconic cores of these thundereggs are even thinner than some of those from the Valley View mine.
A claim was staked sometime before my visit in 1984 by the owners of a rock shop in Radium
Springs. In the shop was a polished slab from a thunderegg. I did not recognize the yellow porphyritic
matrix and blue agate core with tinges of red as being from any location I had in my collection. The slab was
in her collection case and was not for sale. I asked if she had any for sale, bit she didn’t, explaining that too
many were duds. She said I was welcome to go dig in their claim and if I found anything worthwhile, I could
pay her 10 cents per pound. I gratefully accepted her offer and trust. Even if I only found a couple of
mediocre specimens, they would add yet another location to my collection.
The claim had a shallow dozer trench about 75 feet long near the bottom of a set of round hills
protruding out easterly from a range of rhyolite domes making up the Cedar Hills which are aligned north and
south along the strike of the Cedar Hills Fault. The hill appears to be a cap of rhyolite about 75 feet thick,
covering a body of decomposed perlite at the contact, but rapidly grading into a black porphyritic unaltered
perlite about six feet away from the rhyolite contact. This situation did not leave much room for dozing a
trench any deeper than a few feet because of the hard intractable rhyolite on one side, and the equally difficult
unaltered perlite on the other. Hundreds of broken thundereggs were scattered around. It looked like a
sledge hammer was used in an attempt to remove the rhyolite shells from the small agate cores.
This must have been a frustrating enterprise, because broken pieces of rhyolite with agate firmly
attached showed that the agate cores were so completely cemented by silicification of the shells that they
broke unpredictably through the cores instead of popping off, as they do at Valley View, Frieda, and
elsewhere (See Figures 5.01, 5.03). Several large, broken eggs revealed the thin, biconic cores set between
two high-domed caps of matrix making up about 90% of the oval entities.
Severe faulting and deformation affected this deposit after the eggs were filled and the shells had
silicified, because most over eight inches in diameter were fractured completely through. The cracks were
subsequently filled with black manganese dioxide and calcite and conspicuous enough to trace around their
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Robert Colburn: The Formation of Thundereggs
exteriors. The black lines served to indicate which ones to break and where to cleave them so as to salvage
enough agate suitable for cutting into gemstones. Most of these eggs under eight inches in diameter are free
of serious flaws, and should never be broken because the orientation for cutting biconoids can be applied to
these, and some impressive specimens have been produced.
Unlike Valley View, a prominent pressure ridge encircles the narrow girth of the ovoid thundereggs,
but on most of them the ridges are wavy and not in an even plane, that is, the biconic cavities are warped,
probably by some mobility of the flow and plasticity of the lithophysae before solidification. As I discovered,
the cores are so thin that any wavering of the pressure ridge prevents sawing through all of it. This results in
a cut face that shows a rhyolite-to-agate proportion that is related to the degree of the ridge’s departure from
the plane. For example, if the egg is placed in the vise of a lapidary saw so that the pressure ridge on
opposite sides of the egg is aligned with the edge of the blade so as to be making a cut as shown in A to A'
of Figure 5.02, the degree of the rest of the ridge out of that alignment will determine how much of the agate
core can be exposed. If the wavering around the egg is greater than half an inch, less than 50% of the agate
core can be expected to show. The best of these thundereggs do have an even ridge and when properly cut
will show a 70% or greater agate face, belying the fact that these eggs are about 90% rhyolite!
It can be seen that this technique could, and has been used in a dishonest manner to misrepresent a
batch of rough being sold. Generally, the greater the percentage of agate there is in a nodule, the higher its
value. But such chicanery is not necessary, because an honest description of a biconoid’s structure and how
the cut for maximum enhancement was achieved, is more valuable in of itself. If the pressure ridges are
reasonably circular, they are more valuable in their whole geometry to collectors than any gemstone that could
be salvaged by breaking their caps off. When carefully selected Radium Springs biconoids are properly cut
they are quite beautiful, with red-tinged clear-to-blue fortification agate and milky-white centers colored by
microscopic particles of altered perlite clay, all set off nicely in a light yellow rhyolite shell, see Figure 1.05.
Two other locations where commercial attempts to break biconoid cores out of whole eggs should
be mentioned, because of the high quality of the agate, and because I believe a more enlightened approach
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Chapter Five: The How to Determine Core Structures for Correct Cutting Orientation
would yield spectacular, unblemished specimens. One is in Oregon, on the Warm Springs Indian Reservation
near the Deschutes River Canyon. Some very large, broken biconoids with black and brown tube agate
showed up in local shops and shows in the late 1960's. The thundereggs from which they came were said
to be up to two feet in diameter, some weighing over 300 pounds! The highly silicified red rhyolite was
broken off because, according to the story, they were just too big to get out of the canyon whole, and there
was “too much rhyolite around them.”
The large cores were broken and much of the agate had been fractured, but it sold well for a few
dollars per pound to gem cutters who would cut pieces into slabs for making cabochons and jewelry. A few
whole eggs have since been hauled out and cut in half. As this is being written, one about 250 pounds was
available for three thousand dollars, roughly ten times the value of an equal amount in weight of agate broken
out of them!
Similarly, biconoids broken from thundereggs from Janos, Mexico showed up in American shops and
shows for a short time. They consisted of brown to red fortification agate, some with contrasting white
banding and others with amethyst centers. Very few, if any, ever made it across the border whole.
Fortunately, agate nodule collecting has become increasingly a more intelligent endeavor, placing more value
on variety, rarity, and formation than just in quantity for the sake of treasure. By acquiring a knowledge of
the structures and external features of thundereggs, and other kinds of nodules as well, the serious collector
could cut them so as to maximize internal features in a way best for information to be gained about their
history.
It is probably safe to say that millions of agate nodules and geodes have been cut randomly, usually
by how they fit best in the vise of a saw. Articles have been published in lapidary magazines about the
impossibility of telling how to cut nodules without ruining their internal features, so if there is a way of doing
so, a discussion is needed, and further proof will be presented in the next chapter as well as it has been here.
The structure of thunderegg cores can, as has been shown, be determined by pressure ridges.
Biconoids usually present a single ridge through which a cut can be made with high success. Star-core eggs

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Robert Colburn: The Formation of Thundereggs

like those from Priday, Oregon, the latitudinal and longitudinal ridges are so evenly arranged, the tops and
bottoms are so obvious that even the number of star points can be predicted for cutting, see Figure 4.14.
Pressure ridges are not seen on thundereggs from many deposits due to warty exteriors, grainy
porphyritic rhyolites or rhyolitic compositions that impede spherulitic crystallization and star formation.