I visited Sibley Volcanic Reserve on Sunday and was transfixed by this:
The light-colored blobs are amygdules, or fossil bubbles. Many of the lava flows that issued from a small volcano here about 10 million years ago were full of gas bubbles. Later those filled with minerals, and today the amygdules are weathering out. The minerals involved include quartz, its noncrystalline variety chalcedony, and various zeolite minerals. Read a little more about them in this page from my old About.com site.
This view shows a set of stretched-out amygdules that represent bubbles deformed by flowing lava.
And this closeup shows two of them sticking out of the rock surface.
A reminder that collecting rocks is not permitted in any of the regional parks, although you can collect rocks and (non-vertebrate) fossils in East Bay MUD watershed lands and Forest Service lands for personal enjoyment.
Amygdules were given their name by UC Berkeley geology professor Joseph Le Conte in 1878. He surely saw them in these very hills.
I also sought out the labyrinths and found three localities. The first one is on the north side of Round Top, hidden in a sweet spot surrounded by vegetation and butterflies, including this Chalcedon checkerspot (Euphydryas chalcedona).
Oakland Geology 
9 June 2009 at 11:00 pm
Those are great amygdules, I was first introduced to amygdules in field camp at Sunset Crater, Arizona.
9 June 2009 at 11:28 pm
You say amygdules, I say amygdales – US & UK: two nations divided by a common language
10 June 2009 at 9:13 am
But UK geologists are confused, probably because they’re all taught French, in which amygdale means tonsils. Amygdule is based on sound scientific Greek as I explain on About.com.
10 June 2009 at 9:21 am
Go down below the lower end of the deep quarry at Sibley Park and you’ll find amygdules larger than your fist!
10 June 2009 at 10:51 am
I’ve heard the Sibley amygdules describes as opaline, but in my experience it seems straight quartz, not even crypto-crystalline. Maybe most of the good ones have already been dug out. :(
10 June 2009 at 12:44 pm
What is the mechanism(s) by which material infiltrates congealed lava and forms amygdules?
[Thurston, groundwater is everywhere, even in lava beds once they cool. It brings in dissolved minerals and leaves them behind. — Andrew]
10 June 2009 at 4:14 pm
I clicked through to your previous post about the labyrinth up there. You begin one paragraph “What amazes me more satisfyingly…” Sir, this phrase needs to make its way into science and philosophy texts. That’s a great post.
7 January 2023 at 7:07 pm
Hi, Andrew, this is great info. We are doing habitat restoration on the Northside of the Caldecott (www.skylinegardens.org–you can see we’ve cleared out the top of Barberry Peak, which you have visited), and a lot of these amygdules have weathered out, along with the zeolites. Lots of “Berkeley Agates” and then some small pieces that have the blue green look of copper bearing rocks. Do you have any idea when the mineral bearing hot water came through the Moraga Formation, or was it from one eruption to the other? Happy to share our restoration work with you.