Gravelly Range: Onyx Rockhounding in Madison County, Montana

Updated June 2026

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

Quick details

Access: Public area
County: Madison County
State: Montana
Nearest road: Gravelly Range Road
Coordinates: 45.09168, -111.85673

Geology at Gravelly Range

This spot sits on Quadrant Formation, Amsden Group, and Snowcrest Range Group, undivided, mapped as 75m(dolomite-cemented quartzite)| 88m(calcareous siltstone, silty shale, shale, sandstone, limestone, dolomite, dolostone, siltstone)| 57m(limestone, dolomitic limestone, shaly limestone)| 75m(dolomitic sandstone, sandy dolostone, and siltstone) that formed roughly 330 to 300 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: Quadrant Formation, Amsden Group, and Snowcrest Range Group, undivided
Age: 330–300 Ma (Late Mississippian to Pennsylvanian)
Rock types: 75m(dolomite-cemented quartzite)| 88m(calcareous siltstone, silty shale, shale, sandstone, limestone, dolomite, dolostone, siltstone)| 57m(limestone, dolomitic limestone, shaly limestone)| 75m(dolomitic sandstone, sandy dolostone, and siltstone)
Elevation: 8,260 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 Gravelly Range

Best months: May to October. Typical highs run 72°F in midsummer and 25°F in midwinter, with winter lows near 7°F. At about 8,260 ft, snow can block access from late fall well into spring, so check conditions outside of summer.

Climate normals: NOAA 1991-2020 monthlies, ALDER 17 S station (10 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 Gravelly Range

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 Gravelly Range.

Across the state line from Gravelly Range

Gravelly Range is close enough to the Montana border that the next-closest rockhounding spots are in a neighboring state. Worth knowing if you are already on the road.

Gravelly Range FAQ

Where is Gravelly Range?+

Gravelly Range is in Madison County, Montana, at 45.09168, -111.85673. Nearest road: Gravelly Range Road.

What rocks and minerals can you find at Gravelly Range?+

Gravelly Range is reported to produce Onyx.

Is collecting allowed at Gravelly Range?+

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 Gravelly Range?+

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 Gravelly Range?+

The most comfortable months at Gravelly Range are May to October. At about 8,260 ft, snow can block access from late fall well into spring, so check conditions outside of summer.

How do I get to Gravelly Range?+

Open the directions link to navigate to 45.09168, -111.85673 in Google Maps. The nearest road is Gravelly Range Road.

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