The Black Diamond Mines Regional Preserve in the East Bay is a 6,000 acre regional park incorporating 60 miles of trails, 3 deserted mining towns, a 19th century cemetery and a former sand mine, which today is open for guided tours.

The mine opened in the 1920s to produce silica sandstone for the glass industry. Their top customer was the Hazel-Atlas Glass Company plant in Oakland, who later purchased the mine and operated it until 1945.

Tours take 90 minutes and cover 1300 feet of the old sand mine. Reservations are recommended as they frequently sell out. The Greathouse Visitor Center is located underground in a large chamber excavated in the 1920s and features a small museum with displays about the history and geology of the area.