Mount Blackmore: Agate & Chalcedony Rockhounding in Gallatin County, Montana

Plan your visit

Generally open to casual rockhounding

County

Gallatin County

State

Montana

Nearest road

Blackmore Trail

Coordinates

45.44473, -111.00348

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 Mount Blackmore

Comfortable months (45 to 90°F highs)

Best months: May to September. Typical highs run 67°F in midsummer and 19°F in midwinter, with winter lows near 0°F. At about 10,100 ft, snow can block access from late fall well into spring, so check conditions outside of summer.

Climate normals: NOAA 1991-2020 monthlies, BOZEMAN MONTANA STATE UNIV station (15 mi away), adjusted for elevation.

Geology at Mount Blackmore

This spot sits on Absaroka Volcanic Supergroup, Sepulcher Formation, mapped as volcaniclastic rocks, flow breccias, lava flows, welded tuff, laharic breccias, volcaniclastic conglomerate, and volcaniclastic sandstone that formed roughly 56 to 34 million years ago. Petrified wood typically comes from volcanic ash beds like the ones associated with this ground, where buried forests were slowly replaced by silica.

Mapped unit

Absaroka Volcanic Supergroup, Sepulcher Formation

Age

56–34 Ma (Eocene)

Rock types

volcaniclastic rocks, flow breccias, lava flows, welded tuff, laharic breccias, volcaniclastic conglomerate, and volcaniclastic sandstone

Elevation

10,100 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