Rockhounding in Contra Costa County, California

Sketch map

Woof’s favorite find

Mount Diablo Mine

We picked it because its USGS record mentions 2 gem or collectible minerals (Quartz, Agate) and 3 of our card minerals overall. That ties with 3 other site(s), so we broke the tie by mine status (past producers first), then USGS record quality, then name. USGS lists it as past producer for Mercury. Minerals mentioned: Quartz, Agate, Pyrite. Approx. location 37.902, -121.879. USGS record.

Ask first: a USGS record doesn’t mean you can visit. Find out who owns the land and get permission before collecting.

Field cards: what kind of sites?

Status terms come from USGS MRDS records.

Mineral cards

• Silver

  Mohs 2.5–3. Native silver tarnishes black. Mindat

  17 sites

• Copper

  Mohs 2.5–3. Native copper is soft and turns green as it weathers. Mindat

  16 sites

• Gold

  Mohs 2.5–3. Soft, heavy and bright yellow; it doesn’t tarnish. Mindat

  11 sites

• Pyrite

  Mohs 6–6.5. Brassy “fool’s gold,” often cubic. Mindat

  3 sites

• Quartz

  Mohs 7. Six-sided points; smoky quartz is NH’s state gem. Mindat

  2 sites

• Agate

  Mohs 6.5–7. Banded, translucent chalcedony. Mindat

  1 site

Counts are sites whose USGS record mentions the mineral or commodity. Hardness from Mindat.

How we built this page: we counted named sites in the USGS Mineral Resources Data System (MRDS) for this area, left out sand, gravel, crushed stone, clay pits and unnamed sites, grouped them by Census county, and linked every site to its USGS record. We did not visit these sites, and MRDS does not tell you who owns the land or whether collecting is allowed.

Source: USGS Mineral Resources Data System (MRDS). USGS ceased systematic updates to MRDS around 2011, so status and details may be out of date. Each site links to its USGS record.

Updated October 2026