Comments on: Unexpected crystals

By: Andrew

Andrew — Wed, 05 Aug 2009 16:17:14 +0000

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.

By: Mike

Mike — Wed, 05 Aug 2009 05:06:10 +0000

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.

By: Beej

Beej — Mon, 22 Jun 2009 23:54:34 +0000

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. ;-)

By: Andrew

Andrew — Mon, 22 Jun 2009 04:03:31 +0000

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

By: Beej

Beej — Sat, 20 Jun 2009 03:59:53 +0000

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…?