Over my decades as a wandering field geologist in the Panoche Hills, I've seen a lot of strange rocks.

Here's a little about my favorite, and first "personal" gemstone.

 

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The Panoche Hills of Central California were created by the collision, and subsequent subduction, of the Pacific tectonic plate by the North American plate. This collision raised not only the Panoche's, it also caused the creation of most of the familiar mountain ranges and surface features of the Western States. Happily, the subduction event continues to this very day, so who knows what gem treasures, like 'Blue G'  Lawsonite, we may discover on future rock hunts?

'Blue G' is one of the jewelry worthy styles of silicified Lawsonite found only, to the best of my knowledge, in the Panoche Hills of California's Inner Coastal Range. It, and the other styles of Panoche Hills Lawsonite, formed deep in the volcanic subduction melt above the Pacific plate, as it dives below the North American plate. This ongoing plate tectonic/volcanic event is supposed to have created not only the Sierra Nevadas and Coastal Ranges, but many of the geological features of Western North America.

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Lawsonite is commonly found as an accessory mineral all around the Earth, but Panoche Hills Lawsonite is distinctly different from the norm. 


The most common style found in the Panoche Hills is analogous in form to Eclogite, except with Lawsonite as the primary mineral, rather than as a accessory. 

Put simply, it's a garbage can assemblage of ground up sedimentary formations, cemented together with silica, included by lots of fresh blue  Lawsonite
This occured when seafloor sedimentary formations were sucked into the volcanic melt caused by the tectonic friction of the Pacific Plate diving under the North American plate.

Now, Lawsonite in other localities, as it approaches surface conditions, quite literally dehydrates into anorthite feldspar. The difference in the Panoche Hills was the very high percentage, as much as 20% of volume, of water in the volcanic melt. Combine that with high pressures and medium to low temperatures in the melt, and the Lawsonite wasn't able to do as it does elsewhere, metamorphose/dehydrate into feldspar.

 

Additionally, the Lawsonite nodules and seam pseudo-nodules appear to have been highly silicified, or as I say, agatized. They're often found as rounded, gnarly nodules, which can stand up to quite a beating before they break. I suppose that they have a sort of inner tension, much as some Apache tear obsidionites.

Happily, I've yet to come across a academically trained geologist who disagrees with, or disputes my suppositions about my favorite gemstone. 

That's a great thing, since I had the GIA do the basic identification work. 

I paid for it, so I count it as MY gemstone, filling the number one spot on my bucket list!

Links:
https://en.wikipedia.org/wiki/Subduction
https://en.wikipedia.org/wiki/Lawsonite
https://en.wikipedia.org/wiki/Eclogite
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That's it for now, rockhounds. My full article, and all of the info you'll need to go out and rockhound the Panoche Hills and find your own Lawsonite, will be in a forthcoming PDF.

Happy digging!

Kris

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Kris Rowe is a wandering field geologist, prospector, field trip leader, lecturer, writer, community activist, lapidary and small businessman.
He currently resides in Fresno, CA, and spends most of his time trying to herd 3 cats, one beautiful woman, several "grasshoppers," and an ever growing family of rockhounds.

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