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Digital Subscriptions > Rock&Gem Magazine > May 2019 > VOLCANIC GEMS


Fragments of Clear Magma Dot Wine Country

I found it lying in the middle of a dirt road among vast vineyards near Cobb Mountain, overlooking Clearlake, California. It looked precisely like a piece of broken glass from an amber beer bottle, but multi-milliondollar vineyards that make glass litter seems unlikely. I also knew that the road crossed the epicenter of Lake County diamond country.

The specimen I found was a xenolith, formed from molten silicon dioxide contacting a coloring element, somewhat as a glassblower fashioning decorative glass. The high-temperature environment within the magma chamber of Mount Konocti, 1,112 degrees at a minimum, gave it a 7.5 or higher Mohs hardness and allowed absorption of the coloring element. The resulting “diamond” was not merely stained quartz but a gem. Many thousands of years after its origin in a volcanic eruption, the road grader bit deeper into the earth and uncovered it.

My find was an unusual variant of a typically colorless gem, with lavender and yellow specimens predominating. Though visually crystalline, the colorless specimens that lie about freely here are not crystals, which form at lower temperatures, but fragments of clear magma.

A brilliant luster defines these stones. As I walked along, inspecting the graded furrows lining the road, glints of light caught my eye. Only millimeters in size typically, the pure “diamonds” emitted signals of light. The atmosphere, measured as most pollution-free in California, helped raise their visibility. At home, their brightness reminded me of the high-priced Lake County diamonds set in jewelry online.

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Rock & Gem May 2019, Volcanic Views : Remnants of Eruptions, Artisanal Mining : A Complex Industry, HEMATITE : An Influential Mineral Iron Ore, And More.....