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What Is That Green Rock?
July 12, 2003
I found an interesting looking rock out at my farm a while back. Having watched a few programs on the Discovery Channel (and/or its clones) about meteorites, I wondered if my find might be a meteorite. I did some research on the web and assured myself that what I had was not a meteorite, but instead was a "meteor-wrong." It was really just basalt. Well, I didn't pay much attention in Introduction to Geology, and that was 37 years ago, so I wasn't too surprised by my mistake. Then I got to thinking about the various rocks that were lying about at the farm, so I started doing more web-based research about rocks.
One of the most useful sites I found is the Rock Key. You can use this to figure out what kind of rock you're studying. You'll need to read more in that web site than just the Rock Key, because otherwise you'll have problems answering the questions about your rock. You'll need to know what is meant by "large grains," "small grains," layers," "crystals" and other such descriptions. And you'll need to crack open your rock to get to an unweathered surface. That is where a nice big hammer will come in handy. Be sure to wear eye protection.I gathered up a selection of the rocks from the farm and began trying to label them. After much study and gnashing of teeth, I determined that I had: basalt, diabase, gabbro, granite, quartzite, gneiss, diorite, rhyolite, breccia and serpentinite. Of all the rocks I found, serpentinite is the most beautiful.Serpentinite is the official California State rock. It is green and I have found two main forms: a pale milky green form and a darker green form. You can find many web sites with scientific discussions of serpentinite. Use Google, Alta Vista or whatever is your favorite search engine.
Here are the two forms I have found at the farm. On the left is the pale milky green form. On the right is the darker green fibrous form. As I read about serpentinite, I became convinced that it does not naturally occur where the farm is. I picked up all the serpentinite I could find, which amounts to quite a pile of it. I noticed that the samples I found were along a county road that was higher than the adjacent farm land. The rocks must have been laid there in the road bed and they must have been quarried where serpentinite is abundant, perhaps in the nearby Coastal Range. I went and searched along the other county road that borders the farm, but I found very few examples of serpentinite. That road bed must have been quarried somewhere else. So the only reason I have my serpentinite collection is that, by pure happenstance, whoever laid that one road bed got their rocks from a serpentinite-rich location. Serpentinite Examples
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