Jefferson River: Agate & Chalcedony Rockhounding in Madison County, Montana

Plan your visit

Generally open to casual rockhounding

County

Madison County

State

Montana

Nearest road

Primrose Lane

Postcode

59751

Coordinates

45.69019, -112.27452

Coordinates are approximate. Confirm the exact ground in the RockHoundR app, where land overlays show which agency manages the area.

Land & collecting status

Generally open to casual rockhounding

Most public-tagged spots sit on BLM, U.S. Forest Service, or other federal land where reasonable hand collecting of common rocks and minerals is allowed. Confirm posted rules and active mining claims before you dig.

Public-land rules vary by agency, season, and field office. The RockHoundR app pulls live BLM, USFS, NPS, and tribal overlays so you can see exactly which agency manages the ground at this spot.

Best time to visit Jefferson River

Comfortable months (45 to 90°F highs)

Best months: March to November. Typical highs run 85°F in midsummer and 36°F in midwinter, with winter lows near 13°F.

Climate normals: NOAA 1991-2020 monthlies, TWIN BRIDGES station (10 mi away).

Geology at Jefferson River

This spot sits on Alluvial deposits, mapped as sand and gravel that formed within roughly the last 3 million years. Petrified wood typically travels as tough float, and worked gravels like these are exactly where weathered pieces tend to collect.

Mapped unit

Alluvial deposits

Age

3 Ma to present (Pleistocene to Holocene)

Rock types

sand and gravel

Elevation

4,500 ft

Geologic mapping via Macrostrat and the underlying state geological survey maps (CC BY 4.0). Bedrock at the surface can vary within a mapped unit.

Sources & verification

Spot details combine the public RockHoundR location dataset, normalized mineral labels, agency land-status checks in the app, and community submissions. Coordinates are approximate until verified in the field.

State guidance last verified: April 1, 2026

• BLM Montana, Recreational mineral collecting
• Montana Bureau of Mines and Geology
• Gem Mountain Sapphire Mine (fee-dig operator)
• Community submissions reviewed against published agency rules