New Mexico Mineral Symposium — Abstracts

Central New York state is famous for the doubly terminated quartz crystals, known as "Herkimer Diamonds." The town of Middleville, New York, has been the source of some of the highest quality specimens and is the location of two well-known "mines" open to the public for fee digging. During the fall of 2007, the author and Jeff Fast of East Haddam, Connecticut (JBF Minerals), conducted a commercial dig at the Herkimer Diamond mines site in Middleville and extracted 38pockets of quartz. In addition, the author's father (John Medici) and brother (Brett Medici) visited in October and found four pockets in the fee digging area of the mine.

Collecting at this site usually involves removing 8 ft of overburden consisting of fairly tough limestone. Beneath the overburden is a 1-2-ft-thick layer known as the "table," which is highly silicified dolostone (Ulrich 1989) and is extremely tough. The larger pockets are found in the center of this layer and vary from 1 to 6 ft in diameter. One pocket can contain thousands of crystals, from sand grain size to "goonies" the size of grapefruit. The most desirable crystals are in the 1-2 in range and almost perfectly clear. A good rule of thumb is that it takes as long to remove the overburden as it does to lift the table.

As commercial diggers, we dug in a private area of the mine with 4 ft of overburden, which was blasted before the dig. The blast was interrupted by an electrical storm and a curious neighbor in an ultralight flying over the blast area. An excavator removed the blast pile for us, leaving approximately 2 ft of overburden and the table. Our tools consisted of standard sledgehammers, chisels made from jackhammer tips, a hammerdrill ( to help set chisels), and a diamond chainsaw for removing entire pockets. By contrast, the general public is limited to 12-lb hammers and chisels.

Initial conditions were ideal, with dry weather and lots of pockets. Recovered specimens were decent but not spectacular. Removing entire pockets was a challenge. The chainsaw required an operator to stand with one leg in a stream of water and breathe exhaust for the duration of the cut. Invariably, the pocket base would fracture as it was pulled up—we compensated with duct tape and nylon cinches.

As we dug pockets, the location and shape of each was mapped. The pockets are believed to be formed after algae formations known as stromatolites were buried by sediment, which decayed to anthraxolite (a coal-like mineral), and left voids later overgrown with dolomite and quartz (Chamberlain 1988). The map does show spacing reminiscent of plant growth patterns in a shallow sea.

Late in the dig, a wet weather pattern impeded our progress. Many times we had to pump out the commercial dig, which filled with several feet of water. John and Brett Medici visited during this wet period in October and found four pockets using only 12-lb hammers and chisels in the public part of the mine. This talk will document the adverse conditions and determination involved in digging for quartz in Middleville.

The crystals recovered in the commercial dig were typical of Herkimer "diamonds," with a few outstanding ones (see the cover of Rocks and Minerals, May/June 2008 for a skeletal group belonging to Jeff Fast). Late in the dig, Pocket #32 had an abundance of phantom quartzes, each of which had a black core overgrown with clear quartz, which was unusual for Middleville.
 

References:

1. Chamberlain, S. C., 1988, On the origin of Herkimer "Diamonds": Rocks and Minerals, v. 63, no. 6, pp. 454 455.
2. Ulrich, W., 1989, The quartz crystals of Herkimer County and its environs: Rocks and Minerals, v. 64, no. 2, pp. 108-122.