Need rock identification help (red smooth rock)?

I was on Ocean Beach in San Francisco and picked up this peculiar rock. It is small, about an inch, and is polished with smooth edges. It is streaked with a shiny red like "polish" that creates wavy stripes on the otherwise dark gray rock. (Kind of looks like it belongs in someone's rock collection..) I would love to know what kind of rock it is, and what it is doing on a beach! Thanks!