Black Diamond Mines Regional Preserve, Regional mining preserve in Contra Costa County, California.

Black Diamond Mines Regional Preserve is a large protected area north of Mount Diablo featuring grasslands, oak woodlands, and mixed evergreen forests. The site contains old mining shafts, an accessible sand mine, and a visitor center with historical displays.

William C. Israel found coal at Horse Haven Valley in 1859, and the Mount Diablo Coalfield went on to yield more than 4 million tons. The low-grade coal was for a time California's only readily accessible and economic source, fueling railroads, ships and heavy industry. Mining ended as affordable imported coal and petroleum took over, and the towns were abandoned.

The Rose Hill Cemetery holds graves of over 200 residents from different communities, including Welsh miners and their families who worked the coal mines. The headstones tell stories of the people who built these settlements and worked the land.

The preserve offers extensive trail networks and guided tours through historic mining sites, including an accessible sand mine visitors can enter. Wear sturdy shoes and be prepared for varying terrain conditions, especially if exploring caves or steeper sections.

Visitors can walk the first 200 feet of Prospect Tunnel, a former coal mine that is about 400 feet long and kept open by park staff. The preserve also holds Rose Hill Cemetery, where more than 200 coalfield residents are buried, including miners killed at work. In 1960 Ansel Adams photographed two Italian cypress trees there as "Poplars, Cemetery near Mount Diablo".