Three Minute Video on How Geodes Form

Here is a great video on how some agates form, perhaps most:

At this point, let me lapse into text from my now dead book project. This is me talking:

In the case of agates, some literature calls them a, “distinctly banded fibrous chalcedony.” While that may be true for science, a technical discussion on that beyond the scope of this book, agates are simply pretty, translucent at least in part, and common throughout America and the Southwest. I’ve described Burro Creek before. That’s an area in Arizona for collecting. The Cady Mountains in the Mojave Desert near Barstow, California has many kinds of agates, including what’s called the beautiful Top Notch. Clubs may have claims focused on agates, like the one near Las Vegas owned by the Southern Nevada Gem and Mineral Society.

In an interview with Valley Verde TV, Pat McMahan, the world’s leading agate expert, summed up their origin story in just a few unscripted sentences.

“An agate is actually a type of quartz. It’s a non-crystalline quartz and it often forms as a result of volcanic ash like what you have at Mount St. Helens. So, you have a lava flow like you have in Hawaii that has gas pockets and cracks in it — these become the future home of agates. You then have a subsequent eruption with volcanic ash. Rain falls on top of the ash, picking up the silica of the ash, ashes are a form of quartz, depositing that in the lava rock. The rain then soaks through the lava rock, fills the gas pockets, and if it picks up minerals in the process, you have agates that are colored and have inclusions of different kinds. More rain ensues, and ashes are washed away after millions of years, the lava rock turning into soil, with agates just laying there for us.”

Video with Pat explaining geode formation here:


In explaining petrified wood formation, Halka and Chronic use similar terms. “Because volcanic ash is made of tiny fragments of unstable silica glass, groundwater seeping through the sediments soon becomes charged with dissolved silica. The silica tends to come out of solution when it contacts organic material such as old wood or animal bones. Little by little it has accumulated in pore spaces within the trunks, bringing with it traces of iron, manganese, and other mineral substances that now add brilliant color to the wood.” If we can think of gas pockets and cracks filling with silica instead of replacing pore space within wood, we might better understand the process of agate building.

The first video graphically demonstrates what Halka and Chronic and I could only paint a picture of with words.
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Jim Gray’s Petrified Wood Company in Holbrook, Arizona

A monument to petrified wood allthough they have plenty of other material. That stock is not too focused on Arizona rocks and gems. A must stop in Holbrook which is in Navajo County. With all pet wood, test for “UV” and “U” if you can. Here’s what I write in  my travel list:

Jim Gray’s Petrified Wood Company

147 Highway 180
Holbrook, AZ 86025

34°53.433′ N 110°09.581′ W

Self-billed as the largest rock shop in the world, this store may qualify as such. Located on the way to the Petrified Forest National Park, the outside rock yard contains tons of petrified wood, a veritable forest of its own. Inside, polished pieces present themselves in all forms, from table tops to book ends.

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The Rainbow Rock Shop in Holbrook, Arizona

This shop was closed when I visited in spring. I took a few pictures and rolled on. Apologies for not adding it to my travel list. A must stop. Call ahead to make sure they are open. Holbrook is in Navajo County, close to the Petrified Forest National Park.

Rainbow Rock Shop (Indian Rock Shop)
101 Navajo Blvd
Holbrook, Arizona 86025

34°54.080′ N 110°09.528′

Google map link:

Multiple dinosaur statutes out front. Do you need any more encouragement?

Facebook page:

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The Estate Rock Yard of Ed Rupprecht in East Las Vegas

Rupprecht Estate Rock
Care of Jason Fabbi G.G.
East Las Vegas, NV

Appointment only.

[email protected]

Small rock yard representing more than sixty years of collecting in the Southwest by Ed Rupprecht and his wife. Mostly rocks that take a cut and polish, little in collectible minerals. Mostly rough, some slabs. Please bring cash in small bills. Rocks in general are two dollars a pound, slabs more, some things different. Ask Jason for particulars.

Bring your UV lamps, there is a barn like structure on this property with good darkness. I got plenty of pretty lime/green pieces in quartz like rocks, no red or unusual colors. It was not a calcite fest, either, which was good. And I only had a SW lamp, so your luck will be better if your portable lamp has all three wavelengths. Pet wood, opalized wood, a septarian nodule or two, rhyolite, and on and on. Well worth looking at if you are in Las Vegas.

Contact Fabbi to see if the yard is open. Jason is an extremely talented jeweler with a long history of collecting and finishing stones. He is very active in the local club, the SNGMS and does much custom work. He made a handsome bolo tie for me out of gold in quartz that I found.

Visible light

Handheld iPhone photo in SW just to give you an idea. 18 watt Way Too Cool lamp.

Picture of the rock yard.

One more picture.

And a video.

Estate rock yard of the late Ed Rupprecht in East Las Vegas from Thomas Farley on Vimeo.

A great man.