Agatized coral Thunderegg?

[howgneiss]

Would this be classified as a thunderegg or geode or something else?

[jasoninsd]

That's wicked cool!!

Tagging 1dave on this one...

[1dave]

It is hard to be sure from a photograph, but that looks to me like a Coastal Range Thunderegg. They differ from more easterly eggs in having marcasite -

(MIST - Mercury, Iron, Sulfur, and Thundereggs.)

Lots of mercury and sulfur = red cinnabar, Iron and sulfur = pyrite or marcasite on the west coast,

More magnetite, hematite, limonite instead further inland.

Lets check with the master on thundereggs - Richard Paul Colburn, the Geode Kid.

The following quote and picture are from his writings in the Geode Kid Free Club and his CD "The Formation of Thundereggs."

(Page 3 for August, 2002)

Lone Hill was a very unique geologic structure. It sat alone in the flat Santa Clara Valley about a mile east of Blossom Hill Road that skirted the eastern foothills of the sharply rising Santa Cruz Mountains. The hill was a single rhyolite volcano that extruded perlite interspersed with crystalline rhyolite lava about 20 million years ago. It had a cap rock of rhyolite forming a four to six foot rim around the northwest side of the hill otherwise, all other features hid any outcrop of thundereggs. The hill was only about 150 feet high by about one half mile long. It was a classic rhyolitic dome standing alone, a savanna island in a sea of fruit orchards and vineyards. One house was built on the north side near where Union Avenue made a Junction with a road going north. The only other inhabitants was the Mirassou Winery located on the flat adjacent to the west end of the hill. San Jose with 45,000 population in 1950 was out of sight 10 miles easterly. Los Gatos, a country town of 5000, was six miles to the northwest. Apart from that, the only territorial imperative was the symphony of summer bugs and an occasional car pulling into the winery to taste their vintages. I would never see that Lone Hill again.

The Lone Hill thundereggs were rather dull compared to most, but they had their own beauty and as I was to discover, a uniqueness found in all Coast Range thundereggs, and not in any other western American geologic province:

They all contain the sulfide of iron, marcasite.

On the surface, most of the marcasite is rapidly oxidized to a rusty color of hematite and light yellow limonite.

[howgneiss]

I should add that this was found on the shore of Lake Huron.  Thank you for the responses Jason and Dave.

[1dave]

Was it native to the area or dropped by a tourist? Will we ever know?

[amygdule]

Not a Thunder Egg
Ore a Geode
Just a Coral
With an Agate
Center