They aren’t diamonds at all — but few crystals look the part so well. Herkimer “diamonds” are doubly-terminated quartz crystals of remarkable clarity, naturally faceted and pointed at both ends, found in the Mohawk Valley of central New York. At the region’s fee-dig mines, anyone can crack the rock and keep the crystals they find.
What is a Herkimer diamond?
A Herkimer diamond is simply quartz — but quartz that grew freely inside gas pockets in a very old dolostone (a dolomite-rich rock) laid down roughly half a billion years ago. Because the crystals developed suspended in fluid-filled cavities rather than attached to a wall, they formed points at both ends and a naturally faceted, gem-like habit. Their clarity can be extraordinary, and some enclose water bubbles (enhydros) or black hydrocarbon inclusions.
The fee-dig mines
Two long-running operations near Middleville — the Herkimer Diamond Mines and the Ace of Diamonds — open to the public in season. You can surface-hunt the loose material and broken pockets for crystals that have already weathered free, or go hard-rock, swinging a sledge to split the tough dolostone and open fresh cavities. Whatever you find is yours to keep, and the mines rent tools for those who arrive empty-handed.
What you’ll find
The crystals
- Clear doubly-terminated single crystals — the classic
- Clusters & twinned crystals
- Smoky & included stones
- Enhydros — crystals with trapped water
Access & the law
- Fee-dig operations — pay admission, keep your finds.
- Surface sifting is easy; hard-rock work is strenuous.
- Tools available to rent; wear eye protection when hammering.
How to collect
There are two ways to work the mines. The gentle route is to search loose ground — sifting the soft pocket clay and combing the tailings piles for weathered-out crystals. The serious route is hard-rock mining: using a sledge, gads and pry bars to split the dolostone along seams and crack open crystal-lined pockets. Bring gloves and eye protection either way.
Getting there
The mines lie along Route 28 near Middleville, in the Mohawk Valley of Herkimer County, easily reached from the New York State Thruway. Both have parking, facilities and shops on site — this is one of the more family-friendly rockhounding destinations in the Northeast.
When to go
The mines run spring through fall (roughly April to November), weather permitting. Summer is busiest; a weekday or the shoulder seasons mean more room to dig. Confirm hours before you travel.
Plan your trip
Pair this with the wider New York rockhounding guide, and read collecting ethics & the law before you go.
Frequently asked questions
Are Herkimer diamonds real diamonds?
No — they are quartz. The name comes from their diamond-like clarity and naturally faceted, double-pointed shape, not their composition.
Do I keep what I find?
Yes — the mines are fee-dig operations, and every crystal you find on your paid dig is yours to keep.
Do I need to break rock?
Not necessarily. You can sift loose pocket material and tailings for weathered-out crystals. Hard-rock hammering yields the biggest stones but is strenuous.
What tools do I need?
For sifting, a screen and bucket. For hard-rock work, a sledge, gads and pry bars — all rentable on site — plus gloves and eye protection.
Are the mines open in winter?
Generally no. They operate spring through fall; confirm the season’s dates and hours before visiting.
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Written by The Field & Stone Editors. Informational only — confirm mine access, fees and rules before visiting. Published by KEVALEX Group.
FIELD & STONEField & Stone is the American rockhounding field guide — where to find rocks, minerals and fossils across all fifty states. Real localities, the best seasons, collecting law and the rock & gem clubs that keep the craft alive, from the Olympic Peninsula agate beaches to the diamond fields of Arkansas.