Herkimer Diamonds are not true diamonds but are a unique form of quartz crystal found primarily in Herkimer County, New York. Their name comes from their striking clarity and naturally perfect, double-terminated shape, which gives them a brilliance similar to a cut gemstone. Their geological history spans hundreds of millions of years, beginning with the formation of the ancient host rock and culminating in a slow, isolated crystallization process deep within the Earth. Understanding their age requires looking at both the bedrock that houses them and the specific conditions under which the crystals grew.
Identity: What Defines a Herkimer Diamond?
Despite the common name, a Herkimer Diamond is chemically composed of silicon dioxide (SiO2), making it a variety of quartz. Unlike the mineral diamond, which is pure carbon, the Herkimer stone is defined by its characteristic crystal habit. This habit is its double-terminated structure, meaning the crystal naturally forms a distinct point at both ends, unlike most quartz that attaches to a matrix.
The crystals typically display 18 naturally occurring faces, which contribute to their brilliance and sparkle. This natural faceting and water-clear transparency led early miners to give the quartz the misleading “diamond” moniker. Herkimer Diamonds are softer than true diamonds, registering a 7 on the Mohs Hardness Scale, but they are durable enough for use in jewelry.
The Geological Timeline: Determining the Age
The Herkimer Diamond begins with its host rock, the Little Falls Dolostone, deposited during the Paleozoic Era. This rock layer, found in Herkimer County, New York, formed in a shallow sea environment approximately 500 million years ago, during the late Cambrian Period. The Little Falls Dolostone is a sedimentary rock composed of calcium and magnesium carbonate sediments that lithified over time.
While the host rock is dated to about 500 million years, the crystals themselves are younger. Herkimer Diamonds formed within cavities, or vugs, inside this dolostone after the rock had been buried and solidified. Geologists interpret the actual crystallization event to have occurred later, possibly during the Carboniferous Period, placing their age between 300 to 450 million years ago. This time frame means the crystals crystallized long before the Age of Dinosaurs began.
The exact timing of the crystal growth is still a subject of scientific study, but evidence points to a massive time gap between the host rock’s formation and the quartz’s precipitation. This period allowed for necessary geological changes—including burial, temperature increase, and fluid movement—to occur, setting the stage for crystal growth. The age of the Herkimer Diamond is tied not to the deposition of the sediments, but to the later event when silica-rich fluids permeated the ancient bedrock.
The Process of Crystal Growth
The formation of Herkimer Diamonds is attributed to a specific geological mechanism involving hydrothermal fluids. These are hot, silica-rich waters that circulated through the Little Falls Dolostone after it had been deeply buried and subjected to increased temperatures. The process began when acidic groundwater dissolved portions of the dolostone, creating the voids or pockets known as vugs that served as the crystal growth chambers.
Silica, the raw material for quartz, was dissolved from the surrounding rock by the hot fluids and carried into these cavities. As the hydrothermal solutions cooled and pressure conditions stabilized, the dissolved silica began to precipitate and crystallize within the vugs. The double termination, the most distinguishing feature of the Herkimer Diamond, is a result of this isolated growth.
The crystals were suspended in the fluid within the vugs, meaning they had minimal contact with the cavity walls, allowing them to grow freely in all directions. This undisturbed, slow crystallization process resulted in the highly ordered, transparent, and naturally faceted crystals that collectors prize. Inclusions of materials like black hydrocarbons (anthraxolite), liquid saltwater, or petroleum trapped during this growth provide clues about the chemical environment of their deep geological origin.