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Last Updated: 11th Sep 2013

By Dave Crosby

Geode Kid surrounded by his Collection

The premier location in the world to study Amygdules and Lithophysae is the Geode Kid's Collection in Deming New Mexico.


He became infatuated with geodes while in high school and never lost that enthusiasm.

He has dug at every known site in the western United States and personally discovered many of them by pouring over reams of geological maps.He focused on site types and HERE they are!

The collection has been visited by top geologists from Europe and featured on the Travel Channel.

Lithophysae AKA Thundereggs, AKA geodes are found world wide, weathering out of rhyolite or obsidian. Size varies from less than an inch to over 12 feet in diameter.
Their unusual exteriors immediately proclaim you have found something unusual. All thundereggs begin as spherules. More information is given with each image.
Spherule Diagram

Iron Co., Utah, USA
Spherule Transmutations

As the flow mass cools, shrinkage allows the trapped water to partially flash into steam, rupturing the spherules, trying to increase it's volume by 2,000%. How much they rupture is dependent on the amount of overburden. Those near the surface rupture completely, those at a depth greater than 9 feet usually remain spherules.

This Richardson Ranch geode was purposefully cut to show how the original spherule crystal was ruptured by steam.

A train of pristine (never filled) lithophysae is visible on the roadside leading to the Schoo mine 1/2 mile South and above the Blundell Geothermal Powerhouse northeast of Milford, Beaver Co. Utah.

If ruptured spherules are near or below the water table, over time they will be filled with whatever minerals are left behind during each drying season. Filling sequences vary in time and temperature.

A world class collecting area is Richardson Ranch near Madras Oregon. The Priday Plume beds were not yet a part of the ranch when I visited in 1962, but the Blue Beds,Red Beds, and Pie Beds were open, well weathered easy to dig in matrix, and EXCITING!

The outer shells of lithophysae are usually tenacious, but sometimes they fall apart in your hands, leaving you with just the cores. Here on the right are three showing casts of the original cristobalite needles that encased them. Note the "dimples" and "pimples" and needle impressions on the sides.

By far the most exciting Thundereggs I ever dug were at the Spencer Opal mine near Spencer Idaho. They were formed in ruptured spherule filled pitchstone obsidian and varied from 1" to 6" spheres usually expanded to half their diameters and filled with bands of precious opal!

Below are drawings I made to help others understand what to expect from the exterior view of their specimen. Each surface ridge is where a fracture penetrated to the exterior and usually are filled with agate. If steam pressure held within the void until the surrounding material crystallized the ridges remain extruded. If not, they were sucked back in. On further cooling, the voids became vacuums, greedily sucking in any ground waters.
Thunderegg exploded view

Iron Co., Utah, USA
Thunderegg sections

Iron Co., Utah, USA

Fill sequences have recorded a lot of neighborhood history. Voids above the water table remain empty. Those near or below record minerals delivered by liquids hot enough to leave deposits on all faces, plumes grown in jells, broken debris from rock slides or earthquakes, flat layers of opal and agate deposited in cooler times. Often new volcanic activity reheated the region and the fill sequences started all over again.
Talking Thunderegg
Australian Lithophasa


Note - 2 Apr 2013: Mindat member Nathalie Brandes http://www.mindat.org/user-13977.html#2_0_0_0_0__ advised me to get up to date by reading New Mexico Geological Society Special Publication No. 9 (1980) on ash-flow tuffs and zones in welded ash-flow tuffs.

She was right. Back in the fifties the geologists I studied under were only aware of ash FALL tuffs. Ash FLOW tuffs are a very different subject as I soon gleaned from the publication.

Spheroids and lithophysae (Thundereggs) DO exist in welded ash-flow tuffs.
“That that is is. That that is not is not. Is that it? It is. -“Flowers for Algernon” by Daniel Keyes.”
Table Butte Google View
Ground view of the north east end
Ash-Flow Tuff
Welded Ash-Flow Tuff Thunderegg

Some friends and I verified this by visiting Table Butte in western Iron County Utah this past weekend.
Yes, it is composed mostly of ash-flow tuff.
Yes, we found lithophysae that had tumbled down from some higher layer. Those we found contained agate in shallow cavities 4-6" wide, seldom more than 2" deep, but pretty well broken up.
Broken Thundereggs

No, we did not climb up to find the layer they formed in. We will leave that up to the younger generation.
A thought. Is it possible they formed in the welded ash-flow zones where steam trapped in pumice chunks were compressed back into water, allowing surrounding silica the mobility to crystallize as cristobalite spherules, then cooling contraction allowed the water to flash back into steam, opening the cavities?
The cristobalite, unstable at cooler temperatures, could be the source for the agate that later partially filled the cavities.

Article has been viewed at least 28687 times.

Discuss this Article

10th Feb 2016 15:02 UTCDave Crosby

Baker Ranch NM not buried~
From: John Oostenryk
To: Dave Crosby
12/17/2015 09:22PM
Hi Mr. Crosby,
John Oostenryk in NW IL here~ Have appreciated your contributions to Mindat!
We share an interest in forms and deposition of "chalcedony" (such a bland term for cool varying stuff!! Only because I wish to keep this short:)-
Just a quick communique- I have been on the property around the Baker Ranch thunder egg claim pit the last two Springs, (2014 and 2015) and suspect I will be back that way again this 2016 too.
Happy to tell you the pit is not filled in at all. Still fenced and posted no trespassing (I do not access) Nor are the other minor diggings in adjacent view area.
Probably should remove your statement about burial from your image descriptions on site posted in gallery?
Jeffrey Anderson (Dwarve's Earth Treasures website) is a friend of mine. He stays at my place for stopover when driving from AZ to MI when our schedules allow it, to go Laker Agate hunting. He has been helping out at Claim yearly for long time (prior to Paul's passing) and still does when they do the annual mining (not rockhound roundup~) He has confirmed a rumor I have heard elsewhere- that the present digging is looking almost played out as to commercial claim (good material recovered is increasingly scarce:(
I would comment that there are some old pits- practically straight S of Claim, basically just over fence on N side of highway 9. Those have been mostly to completely filled in - as they were hazard to cattle. I figured agate prospects but, At least one was likely copper prospect- I found an insitu, 8" chunk of massive black calcite (Mn? included) shot through with a minor chrysocolla vein. Not sure what story is on those old workings.
I would DEARLY love to have a T-egg with blue chrysocolla plus teh local colors~ or green copper oxides, say with blue opal like from there!! Wild concept isn't it!? Amusing to posit possible combinations inferred from random finds thereabouts... But, no way to know IF or Where.... Get out the dowsing rod!!?? Just kidding that!

Best Regards and Happy Hunting!
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