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Frequently Asked Questions (FAQ)
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There are questions about Herkimer diamonds that get asked often. The most frequent is "Why they are so clear?" and "How did they form that way?" There are other questions that the public seems most interested in knowing and here, on this webpage, we will seek to provide some answers.
Question: "Why are Herkimer diamonds here in New York?"
There is a simple answer to this. This place is where a unique set of geologic conditions occurred in this location (Herkimer county, NY) that have not occurred anywhere else in the world. These are briefly listed on the theory page (click Explanation at the top of this page) and understanding how these features combined to help make the Herkimer district is the more complex answer.
There are other locations that produce small quartz crystals with a similar form, but there is no other location in the world that has the range of features found in the Herkimer district.
Question: "Why are they called Herkimer diamonds?"
This more of a historical answer than a geological one. They are named after General Herkimer and the "diamond" term was given to enhance their marketting, due to their clarity and bright sparkle. But the name "Herkimer diamond" is not an official quartz variety name (like the name amethyst). Thus far there is no research to indicate that the quartz from Herkimer county is chemically or morphologically unique. But the lack of science doesn't change the popularity of the name and its widespread use.
Although other locations that produce small quartz crystals with a similar form, the name Herkimer diamond should be applied only to those quartz crystals found in the Herkimer district. But there are a wide range of quartz habits in the Herkimer district. Is every shape of quartz to be called "Herkimer diamond"? If not what are the parameters to use that will say this one is a Herkimer diamond and this other one is not? This may be simply an academic question as the general public will continue to use the term Herkimer diamond.
Question: "Why are Herkimer diamonds so clear?"
This is not an easy question to answer and there really has not been research applicable to this until recently (post 2000). What is observed in the field is that there is a correlation between very clear crystals and black carbon material, hydrocarbon. The oil migration and seed crystal theory suggests that this hydrocarbon is the residue from oil. In addition field evidence, in the form of skeletal and hopper crystals, suggests that the fluid related to the formation of Herkimer Diamonds was at times chemically super-saturated with SiO2 and that there were sharp fluctuations in this availability and nature of this fluid (see discussion on the skeletal/hopper page). There is also evidence of this solution flucuation in the varied forms that quartz takes throughout the Herkimer District. When such dramatic flucuations occur impurities are likely to occur within the crystal because of changes in the rate at which the crystal faces grow. If the crystal can grow at a constant steady rate despite the fluid/chemical fluctuations then it can result in a clear crystal (provided there are no other impurities in the solution like hydrocarbon flakes, baby floater crystals and bubbles). This steady state crystal growing process is aided by the presence of hydrocarbons (oil/gas) in the solution during the Herkimer diamond growing process, thus the link between the black carbon material and the clear crystals. There is more about this on the science page.
There appears to be a size maximum for very clear crystals - about 4 cm point to point, and more commonly 3 cm. But the real common clear ones are under 2 cm. This again is probably related to solution impurities and fluctuations having more time/volume to cause a "not clear" effect when the crystal is larger.
In addition, it is important to consider the effect of freeze-thaw. Any crystal that is exposed to water that freezes in winter can develop fractures. The open holes that contain Herkimer diamonds, in particular the larger pockets, can be an avenue for water movement and thus prone to the freeze-thaw action. The pockets packed with carbon might be less prone.
Question: "Why are some Herkimer diamonds smoky (brownish) in color?"
There seems to confusion about this in regard to the smoky color found in the occassional Herkimer diamond. There is some folklore suggesting that the smoky color is due to micro amounts of carbon matter in the crystal. In actuality this is a correct answer only for BLACK quartz from the district (not smoky). The smoky quartz is actually caused by radiation bombardment from a small amount of uranium that has accomanied some of the hydrocarbon material, but not all of it (and partly why all of the quartz is not smoky).
Another common notion is that there is an Aluminum impurity involved. Although this might be true for smokey quartz in granitic terrain where the quartz has crystallized out of Aluminum rich magma (like the Conway granite of NH and Pikes Peak granite in Colorado) is it probably not true in the Herkimer mining district. Here, in the absence of aluminum bearing magma, it is more likely that an OH bond is being effected, both in making the smoky color and in color healing processes (changing back to clear/white - which doesn't happen as freely with the Aluminum variety). In the Herkimer diamonds the smoky color is often "patchy". It does not have the same appearance as the crystals found in granitic host rock.
The smoky color phenomenon is scattered and the fact that we can have clear (not smoky) crystals with carbon inclusions suggests that some carbon has no associated uranium (unless there is color healing occurring - described in the pdf document below). The smoky color happened some time after the crystal was formed as did the color healing.
Very scientific pdf document on the smoky color in quartz for an environment like the Herkimer district -> -> -> -> -> ->
W. David Hoisington, Ph.D.
An interesting piece of research that looks at dark quartz containing carbon inclusions from Nevada and the Herkimer area. The main focus was to see if the element C (carbon) was fitting into the quartz (formula = SiO2) structure. It was not. But the study did show that Herkimer diamonds with black inclusions contained carbon with free radicals. It was concluded that the dark color of the "smoky" crystals were due to the carbon, which has led to a false rumor in the field that all "smoky" Herkimer diamonds are caused this way. There is a difference between black and smoky. It is likely that some of the carbon material (not all) contained uranium (another research paper discussed finding platinum in anthraxolite at Fonda, see link below). There is also an old paper showing the occurrence of uranium with anthraxolite ( see link below ).The link between uranium and organic material is fairly common. It is the radiation from the uranium in some carbon that causes the occassional smoky color. But black quartz is due to carbon inclusions or coatings.