Saturday, October 31, 2009

Beach Pebble Fun in Marin County

Spending the past few days in San Francisco with a busy schedule, I'd anticipated this would be the first week since launching Mineral Bliss last February that there wouldn't be a post. It was a pleasant family oriented trip, however, into which my son Alex and I were able to squeeze in a few hours for a hike. As we scanned his trail book to pick one, I saw "semi-precious stones" mentioned in conjunction with the "Rodeo Beach Hike." Parking to reach the trail was directly at Rodeo Beach. We drove there and headed out, making it approximately two hundred yards to near the shoreline. Thereafter, we were rarely off our knees.

The picture atop this page is of pebbles ranging in size from about five millimeters to one and a half centimeters. It includes a number of less common pebbles that I'm reluctant to identify without help. Most of what's green is serpentine from the serpentinite rocks abundant in the adjacent Marin County headlands. These headlands are also rich in chert that can be gray, green, red, blue, or brown.
Other beaches along San Francisco Bay yield similar pebbles. Pictured at left are a few that my son Alex picked up several months ago across the Bay at Baker Beach, which is within the San Francisco city limits. I first suspected that the red and brown pebble at center right could be an agate, but am now convinced that it's chert or possibly jasper. One of Marin County's beaches is known as Agate Beach. From the Web, I'm told that many grains of sand there are from agates, but that marine fossils are a better bet for collecting than agate.

Serpentinite pebbles from both Rodeo Beach and Baker Beach remind me of the "jade" that I recall checking out years ago at Jade Beach along the Pacific a couple hundred miles south in the Big Sur. At Jade Beach, the stones are are much larger. Some are actually boulders. They are said to be jadeite and/or nephrite. I'd enjoy the opportunity to compare the Jade Beach material with some of these pebbles.

3 comments:

  1. actually there is no jadeite in big sur only nephrite. However until recent historical times the only known form of jade was nephrite

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  2. The agates at Agate Beach are called "oil agate." These are tiny, amber-colored pebbles which are translucent to transparent. Some of them have dark spots. These agates are not hard to find. The only place I know of in California where you can find both jadeite jade and nephrite jade is at Clear Creek, in San Benito County. Unfortunately, this area is now closed to rock collectors. I have been to both Pescadero Beach and Jade Cove lately, and the rocks seem quite different from each other. I've only found nephrite at Jade Cove and I've only found petrified whale bone at Pescadero Beach, for example.

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  3. Thanks for contributing and sharing this very helpful information.

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