This was the so-called “grand loop” of the Mount Diablo area. It's also known as “four peaks” or “six peaks.”

Starting from one of the trailheads in Clayton, CA, you hit Mitchell Rock, Twin Peaks, Eagle Peak, Mt Diablo, North Peak and Mount Olympia. The first eleven miles of this are mostly through chaparral forests, filled with nice views and fun trails. Miles twelve and thirteen (from North Peak to the bottom of Mount Olympia) are brutal: nasty overgrown trails with poison oak and lots of time carefully descending steep trails on loose dirt & gravel. With or without poles, it's slow going and a severe fall hazard.

Labeling “Mount Olympia” even a “peak” is a bit of a joke: it's really just a ridge on the way down North Peak. It has a prominence of twenty meters. Really, the major points here are Eagle Peak, Mount Diablo and North Peak.

I'd recommend this trail as a one-time experience and for a long training hike, but there's probably a better route down from North Peak. (I've done this loop twice and don't know that I'll do it a third—even though I love the first 11 miles.)