{"nrhp_refnum":"91001425","name":"Black Diamond Mines","other_names":["Black Diamond Mines Regional Preserve"],"category":"district","location":{"address":"Somersville Rd. SE of Antioch","address_restricted":false,"city":"Antioch","county":"Contra Costa County","state":"CA","lat":37.95682860650007,"lon":-121.85014480665282,"geom_source":"nps_gis"},"listed_date":"1991-10-02","is_nhl":false,"nhl_designated_date":null,"areas_of_significance":["Industry","Historic - Non-Aboriginal","Engineering","European","Social History"],"significance_levels":["state"],"summary":{"what_it_is":"A 2,078-acre mining district in central Contra Costa County near Antioch, California, containing the remains of five coal towns, coal and sand mines, tailings piles, a cemetery and related features, mostly from the 1850s to the early 1900s, with sand mining to 1951.","why_significant":"The nomination argues the district is associated with the state's most important coal mining operations (criterion A). It holds an impressive array of coal mining features and the only surviving example of room and pillar sand mining in California (criterion C). It may also yield information on the history and assimilation of Welsh people in California (criterion D).","narrative":"Black Diamond Mines is a 2,078-acre district in central Contra Costa County, California, where coal was prospected in the mid-1850s. Five towns grew around the pitheads, including Nortonville and Somersville, and the coal boom lasted into the early 1900s, when the last mines closed and the towns were abandoned. High-silica sand was mined here from about 1925 to 1951. Recorded remains number 272 features, including building platforms, cellar holes, a cemetery, large coal tailings piles, brick hoisting footings, sealed adits and vents. Underground workings are estimated at 200 miles of passages and over 1,200 acres of mined-out rooms. The district is significant for its link to California's most important coal mining, for the best array of coal mining features and the state's only surviving room-and-pillar sand mine, and for its potential to inform on Welsh immigrants. It is now a regional preserve used for recreation.","period_of_significance":"1859-1907 and circa 1910-1941","architect_builder":null,"architectural_style":null,"criteria":["A","C","D"],"status_note":"Now used for outdoor recreation as Black Diamond Mines Regional Preserve; many mine openings have been sealed or reconstructed, and the townsites are deserted.","vacant_flag":null,"machine_generated":true,"model":"claude-sonnet-5-5","prompt_version":"v1.0-2026-10-29","generated_at":"2026-10-29 02:54:27.303946-05","source_pdf":"https://s3.amazonaws.com/NARAprodstorage/lz/electronic-records/rg-079/NPS_CA/91001425.pdf"},"photo":{"url_800":"https://nationalhistoricalregister.com/media/91001425/c43116d28b-800.webp","url_1600":"https://nationalhistoricalregister.com/media/91001425/c43116d28b-1600.webp","licence":"CC BY-SA 3.0","author":"Sanfranman59","credit_html":"<a href=\"https://commons.wikimedia.org/wiki/File:Black_Diamond_Mines_(Antioch,_CA).JPG\" rel=\"noopener\">Sanfranman59</a>, <a href=\"https://creativecommons.org/licenses/by-sa/3.0\" rel=\"license noopener\">CC BY-SA 3.0</a>, via Wikimedia Commons","source_page":"https://commons.wikimedia.org/wiki/File:Black_Diamond_Mines_(Antioch,_CA).JPG"},"district":null,"sources":{"nps_snapshot_date":"2026-05-22","nara_catalog":"https://catalog.archives.gov/id/123858507","wikipedia":"https://en.wikipedia.org/wiki/Black_Diamond_Mines_Regional_Preserve","wikidata":"https://www.wikidata.org/wiki/Q4920660"},"indexable":true,"url":"https://nationalhistoricalregister.com/california/contra-costa-county/black-diamond-mines-91001425/"}