Ace of Diamonds Mine, Middleville

Check before you go, because the gate is shut. The operator says the Ace of Diamonds mine, campground and gift shop are closed until further notice, and expects that neither camping nor mining will be available for the entire 2026 season. The notice follows what it calls two long years of litigation, during which a preliminary injunction halted commercial operations; later appeals lifted part of that, but nothing has reopened. The valley is still worth the drive: Herkimer Diamond Mines sits a few hundred yards up Route 28, Crystal Grove is at St Johnsville and Diamond Acres at Fonda, and all three are running.

When this one does run, it is one of the classic Mohawk Valley digs. Admission has been fourteen dollars for anyone eight or over and seven dollars for children four to seven, with three and under free, seven days a week from the first of April to the thirty-first of October, nine in the morning until five. Hammers, chisels, garden tools and buckets rent for a dollar apiece, a sledge for four, a sluice kit for six. Whatever you turn up is yours to take home at no extra cost.

What you are hunting is not a diamond. Herkimer diamonds are quartz, seven on the Mohs scale, almost always transparent and running from colourless to smoky. The shape is what makes them worth the work: they grew with very little or no contact with the rock around them, so they carry a termination at both ends rather than one. The host is the Cambrian-age Little Falls Dolostone, laid down about 500 million years ago, and the crystals sit in cavities within it alongside calcite, dolomite, pyrite and sphalerite. They run from a couple of millimetres to over twenty centimetres.

There are three ways people work this ground. Find and break, swinging a hammer at likely dolostone. Scavenging, which means picking over the rubble other diggers have already turned. And cavity prospecting with sledge hammers and wedges, which is the hardest labour of the three and where the best pockets come from.