About this location
The Portsmouth Mine sits on State Highway 6, in a wayside about 0.4 miles north of Crosby, Minnesota, overlooking Portsmouth Mine Pit Lake — at over 450 feet deep, the deepest lake entirely within Minnesota. It's one of many iron and manganese open-pit mines that operated along the Cuyuna Range from 1911 to 1978, in a district famous among rockhounds as the exclusive source of binghamite (also called silkstone or cuyunite), a chatoyant, tiger's-eye-like variety of chalcedony found nowhere else on Earth. The pit lake is now part of Cuyuna Country State Recreation Area, run by Minnesota DNR — great for fishing, scuba diving, and mountain biking, but not for collecting. Minnesota's state parks and state recreation areas prohibit rock collecting outright, so despite the Cuyuna Range's fame for binghamite, this specific site is not a legal place to hunt for it.
Always seek permission where required, follow local laws, and practice Leave No Trace. Locations and details are community-submitted and may be inaccurate. Not sure what's legal here? Read our full guide to rockhounding laws by land type.
Found Here
- Binghamite (silkstone/cuyunite) — a chatoyant chalcedony variety unique to the Cuyuna Range; regionally famous, but not legally collectible within this recreation area.
- Agate, chalcedony, jasper — general silica-family minerals associated with Cuyuna Range iron formations.
- Manganese, iron minerals — reflect the district's historic ore production.
Difficulty
Getting there
The Portsmouth Mine wayside sits on MN-6, about 0.4 miles north of Crosby, in Crow Wing County. It's within Cuyuna Country State Recreation Area and easily reached by paved road.
Collecting here
Not applicable — collecting is prohibited at this site. If you're pursuing binghamite specifically, it's traditionally found outside the state recreation area boundary — on private land, road gravel, or old tailings piles elsewhere on the Cuyuna Range — always with landowner permission, never inside the park.
Know before you go
Rock collecting is not allowed in any Minnesota state park or state recreation area, and Portsmouth Mine Pit Lake is officially part of Cuyuna Country State Recreation Area, managed by the Minnesota DNR. Visiting to see the lake, fish, dive, or bike the trail system is fine; taking rock or mineral material is not. See the DNR's park page for current rules: dnr.state.mn.us/state_parks/cuyuna_country.
Location on Map
Coordinates: 46.49079, -93.97144
Nearby rockhounding locations
- Arco Mine — Agate, ChalcedonyMN · 1.5 mi
- Brainerd — Lake Superior AgateMN · 11.5 mi
- Brainerd — FossilMN · 14.1 mi
- Little Falls — Lake Superior Agate, GarnetMN · 38.2 mi
- Charles Lindbergh State Park — StauroliteMN · 41.9 mi
- Royalton — StauroliteMN · 49.5 mi
More Agate locations in Minnesota
Frequently asked questions
Can you collect binghamite at Portsmouth Mine in Minnesota?
No. Portsmouth Mine Pit Lake is part of Cuyuna Country State Recreation Area, and Minnesota DNR rules prohibit rock collecting in all state parks and state recreation areas, regardless of how famous the local mineral is.
Where can you find binghamite on the Cuyuna Range?
Binghamite is unique to the Cuyuna Range near Crosby, Minnesota, but legal collecting means looking outside state park/recreation area boundaries — on private land, roadside gravel, or old tailings with landowner permission — not within Cuyuna Country State Recreation Area itself.
What is Portsmouth Mine Pit Lake?
Portsmouth Mine Pit Lake is a flooded former iron-mining pit near Crosby, Minnesota, over 450 feet deep — the deepest lake entirely within Minnesota — now part of Cuyuna Country State Recreation Area and used for fishing, scuba diving, and camping.
⚠️ Always verify current regulations, weather conditions, and access requirements before visiting any location. Information provided is based on community submissions and may not be current or accurate.
Sources
- https://en.wikipedia.org/wiki/Portsmouth_Mine_Pit_Lake — location, depth (450+ ft, deepest lake in Minnesota), confirms it's part of Cuyuna Country State Recreation Area.
- https://en.wikipedia.org/wiki/Binghamite — confirms binghamite is exclusive to the Cuyuna Range, Crow Wing County, Minnesota.
- https://en.wikipedia.org/wiki/Cuyuna_Range — district mining history (1911–1978), Kennedy/Rowe mine openings, Milford Mine disaster (1924).
- Minnesota DNR, "Geology in state parks" (dnr.state.mn.us, via search synthesis) — confirms rock collecting is prohibited in all Minnesota state parks and state recreation areas.
- https://www.dnr.state.mn.us/state_parks/cuyuna_country (via search synthesis) — official park page reference.
Recommended Gear for Minnesota
Estwing E3-22P Rock Pick
The industry-standard 22oz geological hammer. One-piece forged steel with shock-reduction grip.
Clear Anti-Fog Safety Glasses (4 Pack)
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Watertight Medical Kit
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Northwest Treasure Hunter's Gem and Mineral Guide
Whether you're digging for the first time or are an experienced rockhound or "prospector," with a simple rock hammer and...
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