Manhattan: Onyx Rockhounding in Gallatin County, Montana

Updated June 2026

Manhattan is a mapped rockhounding spot in Gallatin County, Montana. Reported finds include onyx. Below: coordinates, access notes, nearby spots, and trip-planning links.

Quick details

Access: Public area
County: Gallatin County
State: Montana
Nearest road: Horseshoe Cottonwood Road
Coordinates: 45.90928, -111.40563

Geology at Manhattan

This spot sits on Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations, mapped as limestone, dolostone, and black shale that formed roughly 360 to 330 million years ago. Carbonate rock like this dissolves and weathers readily, so fresh road cuts, quarries, and creek exposures are typically the most productive ground.

Mapped unit: Madison Group: Mission Canyon and Lodgepole Formations; or Castle Reef and Allan Mountain Formations
Age: 360–330 Ma (Mississippian)
Rock types: limestone, dolostone, and black shale
Elevation: 4,670 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 Manhattan

Best months: April to October. Typical highs run 83°F in midsummer and 31°F in midwinter, with winter lows near 8°F.

Climate normals: NOAA 1991-2020 monthlies, LOGAN LANDFILL station (2 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 Manhattan

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

Onyx

Nearby rockhounding spots

Other rockhounding spots within driving distance of Manhattan.

Manhattan FAQ

Where is Manhattan?+

Manhattan is in Gallatin County, Montana, at 45.90928, -111.40563. Nearest road: Horseshoe Cottonwood Road.

What rocks and minerals can you find at Manhattan?+

Manhattan is reported to produce Onyx.

Is collecting allowed at Manhattan?+

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 Manhattan?+

A rock hammer, sturdy gloves, safety glasses, and a collecting bag cover the basics here. Add a spray bottle: wet rock shows color and pattern that dry rock hides.

When is the best time to visit Manhattan?+

The most comfortable months at Manhattan are April to October.

How do I get to Manhattan?+

Open the directions link to navigate to 45.90928, -111.40563 in Google Maps. The nearest road is Horseshoe Cottonwood Road.

More rockhounding in Montana

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Open Manhattan on the map

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