Herkimer Diamond Mines

Look inside one and you may find seawater, petroleum or carbon dioxide sealed in since the Carboniferous.

Herkimer diamonds are double-terminated quartz crystals that grow in cavities in dolomite around Herkimer County, New York, and the Mohawk River Valley. They are not diamonds. The name refers to their clarity and their well-formed faces, and the classic shape is an 18-sided doubly terminated hexagon.

The inclusions are the real prize for anyone curious about how they formed. Trapped inside these crystals are solids, liquids and gases: salt water, petroleum, and most often carbon dioxide, along with two-phase and three-phase inclusions and negative crystals. The commonest solid inclusion is a black hydrocarbon. That waxy organic material, with silicon dioxide and pyrite, was a minor constituent of the original dolomite and calcite rock. As sediment buried it and temperatures rose, quartz grew in the cavities very slowly, and slow growth is why the clarity is exceptional.

The timing is worth getting right. The dolomite of the Little Falls Formation is Cambrian, laid down about 500 million years ago in a shallow sea taking sediment off the ancient Adirondacks to the north. The quartz inside its vugs is interpreted to have formed much later, during the Carboniferous. The rock and the crystals in it are separated by a very long gap.

For where to actually dig, the Collector's Guide to Herkimer Diamonds by Michael R. Walter is the standard. Reviewing it for Gems and Gemology, Guy Lalous singles out an appendix of maps and GPS coordinates covering the localities open to the general public, Herkimer Diamond Mines among them, alongside Ace of Diamonds, Crystal Grove, Diamond Acres and Hickory Hill. It also documents closed mines and sites outside the traditional Herkimer diamond region, and runs 210 colour images over 96 pages.

One caution about labels. Double-terminated quartz crystals occur worldwide and are sold as Herkimer diamonds regardless of origin. The ones you crack out of the dolostone yourself are the ones you can be certain about.

What it costs, as the mine posts it for 2026: nineteen dollars on a weekday for anyone thirteen or over and twenty-two at the weekend, sixteen and eighteen for ages five to twelve, free under four. Opening was the twelfth of April and the season runs to the thirty-first of October, nine to five, seven days a week, no reservation needed. The ticket covers the whole day and includes collection bags and the use of a rock hammer. Nothing heavier than twelve pounds and no powered equipment. Under-sixteens need an adult with them, and anyone digging more than a week at a stretch is asked to rotate to a different area each week. Everything you find is yours. The mine closes in unsafe weather, so snow, sleet, hard rain or a thunderstorm can end the day early.

One thing to know before you plan a Route 28 trip around both: the Ace of Diamonds next door is closed until further notice and its operator expects to lose the whole 2026 season.