QUESTION:
What is your favorite rock on earth? Have you ever seen Lake County 
Diamonds?  They are beautiful, and found only here, in Lake Co. 

ANSWER from Jack Farmer on February 26, 1997:
I think my favorite rock is actually a metamorphic type, called "eclogite"
(pronounced eck-low-jite).  Metamorphic rocks like eclogite form when
pre-existing rocks are heated up and /or put under pressures different from
those that prevailed when they were initially formed.  The pre-existing
rocks can be either igneous rocks (those formed when molten rock or "magma"
cools and hardens below the surface) or sedimentary (those formed from
broken up or dissolved materials from pre-existing igneous or metamorphic
rocks).

Ecolgite formed when basalt (a dark, iron-rich volcanic rock) was buried so
deeply below the surface that it recrystallized to form a whole new group
of minerals.  This mostly happens  in places where the seafloor (which is
composed of basalt) is forced down into the Earth's mantle by a process
called subduction.

Subduction is a type of plate tectonics that occurs where one plate is
forced down beneath another plate as they come together.  At the Earth's
surface this usually results in uplift to form mountain ranges and/or the
formation of magma which erupts at the surface as volcanoes.  Well this is
exactly what happened here in California about 20 million years ago or so
when the Pacific Plate (which includes most of the Pacific Ocean) was
forced to descend beneath the North Amercan Plate (the one that includes
most of North America).

In California, convergence of the two plates resulted in a lot of Pacific
seafloor basalt being dragged down deep into the Earth where it was turned
into various types of metamorphic rocks, including eclogite.  Eventually
the buried seafloor rocks popped back up at the surface as the Coast Ranges
of California were formed.  Today, scattered around the mountains of the
northern California coast range  you can find isolated blocks of eclogite
(they are kind of rare) formed by the process described above.

Eclogite is kind of spinich green and has little red garnets scattered
through it. It looks much different from the fine-grained, black basalt
from which it formed!  I call it "Christmas tree rock" because it has the
color of a fresh, green Christmas tree covered with red ornaments.

I guess what we can learn from eclogite is simple: Each rock, even rather
dull-looking ones, have a story to tell. In the case of eclogite, it's an
amazing story of volcanic eruptions, crunching plates and uplifting
mountains.  I also like to think of eclogite when I feel a lot of pressure
in my work.  From the lowly seafloor basalt, a beautiful new rock was
created by all that pressure of crunching plates!  In other words the
pressure of hard work does pay off, sometimes in beautiful ways!

Now, about Lake County Diamonds, I saw them once at a rock show, but have
never collected any myself.  I would love to have one for my collection.
And in exchange I'll see if I can locate a piece of eclogite and a basalt
to send to you and your classmates (please include an address where I can
send a sample).  Thanks for the message and I hope you enjoyed the story of
eclogite!
 - Jack Farmer