At the end of a week full of science, I had a touristic trip into the woodlands of Muir. Together with a geologist I met in San Francisco, I traveled to the forest with the large trees seen in the very famous bike chase scene in Star Wars on the second moon of Endor. It was a great trip and I can recommend it to anybody visiting the City of Fog.
The Muir Woods are situated in a valley, where the climate is much different than the surrounding mountain lands. So a geologist and a geophysicist could not resist to climb a mountain to the top (I say mountain, but they were large hills, except when I said that the giftshop woman gave me angry eyes). We decided to follow the Hillside trail, followed by the Ben Johnson trail. These trails gave us a good view of the forest and let us examine some really nice rock outcrops of the mountain. And on one of them I found this rock:
It is a greenish rock, which according to the geologist, could be serpentinite or an olivine containing rock (which I know now, serpentinite is as well, sort of :) ). Me, not being a geologist, wanted to know how he could deduce this. So I asked my fellow geologists I had met in Copenhagen (see this post for some details, not a lot). I posted the figure above and the following photo in our Facebook chat session (very modern, right):
This photo is a zoom-in of the crystal structure. My friendly-geologists from around Europe, started asking me questions about, what is the cleavage, the hardness and does it feel oily like? So I did a hardness test. My fingernail could not scratch it, so the rock hardness is >2.5 (on the Mohs scale), but a copper coin could make scratches on it, so its hardness is <3.5. The cleavage looked conchoidal according to some of the geologists (I'm still not sure how they could see this). Furthermore, I told them that the rock felt smooth, almost oily like. Therefore all the geologists in the chat group decided it should be SERPENTINITE!!! (a metamorphic rock originally containing a lot of olivine).
This tells me that, if you don't know something, ask your network!