Weird backyard find

[RCD]

Okay, when I say backyard, actually a creek bed just over the backyard fence. The creek is a main drainage of the Northwest flanks of Mt. Diablo in the SF Bay Area, CA, USA.  Every winter rocks and boulders wash down the creek, so I cannot know the specific geological time zone, but we've found other fossils here (future posts coming). Looks like it could be fossil to me, and my imagination leads to thoughts of maybe fish? or mollusk? Figured it'd be worth a shot posting this up to see if anyone sees something identifiable. Thanks in advance! RCD

[DPS Ammonite]

Not a fossil. It is a piece of serpentinite with slickensides which is altered lower crustal and upper mantle igneous rocks such as peridotite.