Unexpected crystals


You don’t see a lot of crystals in Oakland rocks. But stopping to examine the boulders in the walls at 166 Tunnel Road, just over the Berkeley line along the Hayward fault, I was arrested by this perfect quartz crystal in a coarse-grained marble block. It’s maybe a centimeter long.

5 Responses to “Unexpected crystals”

  1. Andrew Says:

    Funny that you commented just now, because yesterday I visited this spot again. This big stone, maybe 2 x 2 feet, is the only one of its kind in the wall; the rest is a dull fractured sandstone and some heavily veined rock in which the veins have emerged by erosion; also a block of serpentinite. I still believe this is marble by the hardness and opacity of the white mineral. It might as easily be dolomite as calcite. Calcareous rock is not unknown in the Franciscan.

    It is definitely not the local stone, but I think it must have come from a nearby quarry, nearby meaning the East Bay.

  2. Mike Says:

    Test the “marble” with acid, I think it looks like milky quartz. Milky Quartz is usually with Granite. How hard was your steel, quartz is 7. Marble is softer than copper. Every marble I’ve seen has been a more continuous field, quartz tends to go into irregular crystals, sometimes you get the hexagons, not very often.

    The closest Granite I can think of is Bodega Head (Pacific Plate), or transported from Sierra Nevada mines.

    But then again, I’m just a wanna-be Geologist.

  3. Beej Says:

    Being a caver, I’m always interested in limestone and marble. :-) I don’t know of an east-bay source of the stuff, though there are a few quarries in the north bay. If you haven’t been to the Olema Lime Kilns, it’s worth a trip!

    In any case, maybe I’ll run up Tunnel Rd with splash of vinegar and see if it reacts. ;-)

  4. Andrew Says:

    Pretty sure—steel scratched it. I’ve never seen a rock like it locally, but then I haven’t seen everything around here.

  5. Beej Says:

    Marble? Are you certain? What a trip to have a quartz crystal in it… I presume this was a piece that was brought in to build the wall…?

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