In 1995, the Joaquin Murrieta Chapter 13 of E Clampus Vitus dedicated Black Diamond. The Ore Cart monument is a tribute to Black Diamond and the miners who extracted the coal from the Mt. Diablo foothills from 1860 to 1911.
Sources:
https://www.pittsburgca.gov/our-city/art-about-town
https://ecv13.org/antiochpittsburg/
https://pittsburgmuseum.org/history/f/coal-mining
storymaps.arcgis.com
92 Black Diamond St, Pittsburg, CA, 94565