little falls dolostone case
quartz quartz The minerals of the Little Falls Dolostone are among the most famous in New York State. The Little Falls Dolostone is a rock of Cambrian age that formed at the bottom of a shallow sea about 495 million years ago. It crops out south of the Adirondacks in the Mohawk Valley in Herkimer and Montgomery counties. The material that would later form the beautiful crystallized minerals in this display case was probably deposited as a waxy organic material along with quartz sand and masses of pyrite formed by bacteria. All of this was encased in the rock that is made up of two carbonate minerals, dolomite and calcite.

As the Little Falls Dolostone was slowly buried by new sediments, the temperature slowly rose from perhaps 20 degrees C to 175 degrees C or slightly higher. This burial took about a 200 million years and reached a maximum depth of approximately 5 kilometers about 300 million years ago. As the temperature rose, molecules from the organic material that had been holding in solution the quartz dissolved from sand were broken apart so that the quartz could form crystals. Because burial was extremely slow, formation of the quartz crystals was also extremely slow allowing many to grow clear and gemmy which is why they are called Herkimer Diamonds or Little Falls Diamonds.

The other minerals formed before, during and after the slow crystallization of the quartz. Dolomite and pyrite often formed earlier while calcite sometimes formed at the same time as the quartz, but more often calcite sometimes formed at the same time as the quartz, but more often afterwards. The hydrocarbon black spots in the quartz crystals, the black tar-like material in the cavities, and the microscopic grains that make some of the quartz crystals brown or black are all remnants of the fossil organic material that was essential to the formation of the unusually clear and flawless crystals.

calcite calcite
calcite quartz
quartz calcite
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