Choosing what type of rock is the trick. This is California; we have a great many geological environments, and many rock types could be described as "typical". We live next to the Sierra Nevada batholith, so a big chunk of granitic rock could be used. But that's kind of plain. We are in the Great Valley, so we could go with a big piece of sandstone or siltstone from the Great Valley Sequence, but such rocks are not very durable, and the rocks we choose may very well be in place fifty years from now.
We decided to seek out two rocks that are found very close to our campus, in the Sierra Nevada foothills. The first is serpentine, the metamorphic product of bringing the Earth's mantle peridotite up through the crust under high pressure to the surface. It is relatively rare in most places, but is common throughout much of California. It is unique enough that it was declared our state rock in the 1960s, joining our state mineral gold, and our state gemstone benitoite.
The other problem with serpentine is that on the journey from the depths of the mantle to the Earth's surface, the rock is internally sheared and fractured. It is not always durable enough. So we were invited to search the quarry for a good piece, one that could survive a bit of rough handling. Looking at the steep quarry face, and all the talus that has fallen from the edge, a possible candidate emerged. Look at the photo above (note Mrs. Geotripper up on the side of the quarry for scale): are there any chunks of rocks that have fallen and survived the impact? Yup, I saw it too, looking a bit like a turtle shell.
Here it is up close (below). Not just a big chunk, but one that is covered with the beautiful polished green surfaces that resemble jade. Just the perfect rock for school kids to touch when they are walking into the entrance of the museum. We discussed price with the quarry operators.
"How much per pound?"
"Oh, we sell by the ton. Say $30 per ton."
"We'll take two!"
Then we were off to the Carson Hill Mine Quarry to seek our next objective, a much rarer prize: A big piece of mariposite. Mariposite is a mineral that is practically unique to our region (Mariposa is the Spanish word for "butterfly"; it is the name of the southernmost town in the Mother Lode). It is a chromium bearing mica that is somewhat related to fuschite, but the color is a very bright apple-green. It is often closely related to the gold-bearing quartz veins of the Mother Lode, and I have seen some beautiful samples that contain visible gold. It's a bit harder to find large samples, but the Carson Hill Mine, a Gold Rush era gold mine turned stone quarry, said they had some candidates we might like.
|Photo by Mrs. Geotripper|
Our second mission was accomplished. Two multi-ton boulders collected! Of course, I couldn't resist collecting a slightly smaller piece of serpentine for the department...
|Photo by Mrs. Geotripper|