Rockhounding in Marin County, California

Sketch map

Woof’s favorite find

Union Gulch

We picked it because its USGS record mentions 3 gem or collectible minerals (Quartz, Gold, Silver) and 5 of our card minerals overall. No other named site here scores as high on both counts. USGS lists it as past producer for Copper. Minerals mentioned: Quartz, Gold, Pyrite, Copper, Silver. Approx. location 37.972, -122.732. 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

• Gold

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

  13 sites

• Copper

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

  7 sites

• Silver

  Mohs 2.5–3. Native silver tarnishes black. Mindat

  7 sites

• Pyrite

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

  5 sites

• Quartz

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

  1 site

• Galena

  Mohs 2.5–2.75. Heavy, silvery lead ore that breaks into cubes. Mindat

  1 site

• Scheelite

  Mohs 4.5–5. Tungsten mineral that glows blue-white under shortwave UV. 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 USGS ceased systematic updates to MRDS around 2011, so status and details may be out of date.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