This is a ride I did many times while I lived near the base of Mt. Diablo. It starts at the south gate entrance to the park where some single tracks takes you north from the main road. A mile or so later you connect with a fire road (Dan Cook Trail) which takes you up a steep climb to Rock City where you'll get back on the main paved road up to Curry Point and back on the fire roads. Stick to the left at the first intersection and then keep making your way east. Don't worry about the small section of private property in which you'll eventually reach a wide open area where the trail will split...head up the hill to the gate and jump it to start your decent. At Finley Rd, the dirt ends and the rest of the ride is on the pavement (Tassajara to Blackhawk Dr to Mt. Diablo Scenic Blvd) to end the loop.
Take 680 to Danville and exit at Diablo Bl. Go east 'til you get to Mt. Diablo Scenic Bl. Park about a half mile up the road...trail head will be on the left side at the gate.
http://www.singletracks.com/item.php?c=1&i=778