Stewartville Trail is a 8.4 mile moderately trafficked loop trail located near Antioch, CA that features beautiful wild flowers and is rated as moderate. The trail offers a number of activity options and is accessible year-round. Dogs and horses are also able to use this trail.
I did most of this trail as a part of the "9.4 mile" hike from the Black Diamond main page on ebparks.org. You start on Stewartville, to Star Mine, to Lower Oil Canyon/Oil Canyon/Upper Oil Canyon and finally you have the option to ascend 0.3 miles to the lookout point. Highly recommended.
This is the longest trail in Black Diamond Mines. You get to see a lot of grate views. There's a cave, back pack camping spot, cow corral, and you get to see were the old town Stewartville once stood. If your hiking or walking take the Old Miners trail up (really cool trail). Gravel road to the cow corral thin its dirt road to Somersville road.