It looks like acacia tree seeds - do you have any such tree around? They sprout pretty easy in my zone 5 - I pull them out every year. One tree is enough for me)

Here is a link that might be useful: {{gwi:73347}}

I agree that it looks like a tree seed, although I would suspect a fruit tree, which would explain how the seeds are being dispersed (possibly by animals eating the fruit). In California, there could be a lot of exotic fruit trees to choose from. If there are loquats nearby, that would be my first guess. I was fortunate to live near a property with a lot of loquats when I lived in SoCal, they are wonderful eaten fresh, and the seeds resemble those in the photo.