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

Updated June 2026

Mount Blackmore is a mapped rockhounding spot in Gallatin County, Montana. Reported finds include agate, chalcedony, hyalite opal, petrified wood. Below: coordinates, access notes, nearby spots, and trip-planning links.

Quick details

Access: Public area
County: Gallatin County
State: Montana
Nearest road: Blackmore Trail
Coordinates: 45.44473, -111.00348

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.

Best time to visit Mount Blackmore

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.

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.

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

Found at Mount Blackmore

Each chip opens all spots that produce that material; the encyclopedia link opens the full ID and field guide.

Nearby rockhounding spots

Other rockhounding spots within driving distance of Mount Blackmore.

Mount Blackmore FAQ

Where is Mount Blackmore?+

Mount Blackmore is in Gallatin County, Montana, at 45.44473, -111.00348. Nearest road: Blackmore Trail.

What rocks and minerals can you find at Mount Blackmore?+

Mount Blackmore is reported to produce Agate, Chalcedony, Hyalite Opal, Petrified Wood.

Is collecting allowed at Mount Blackmore?+

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. Always confirm current rules with the managing agency before a trip.

What tools should I bring to Mount Blackmore?+

A rock hammer, sturdy gloves, and a padded bag cover petrified wood hunting. Most pieces are surface float, so walking slopes and washes beats digging.

When is the best time to visit Mount Blackmore?+

The most comfortable months at Mount Blackmore are May to September. At about 10,100 ft, snow can block access from late fall well into spring, so check conditions outside of summer.

How do I get to Mount Blackmore?+

Open the directions link to navigate to 45.44473, -111.00348 in Google Maps. The nearest road is Blackmore Trail.

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