Tennessee Field Diamonds
A treasure 485
million years in the
can they tell?
if anything, can we say about these crystals?
it seems that the ones in the Knox Group formations have
greater variety. Theres always both elongate
prismatic crystals and bipyramidal crystals, and
everything in between. Transitional forms or
scepters, with most commonly clear
bipyramidal quartz growing over milky fractured prismatic
quartz are common. With the one exception, the
occurrence near the Great Smoky Fault at Walland, no
Sevier Shale crystals are strongly prismatic, and none
show scepter overgrowth formation. In the next
slides we will see if any conclusions can be drawn.
This is the Nakaya Diagram, presenting experimental
results from the first experiments to grow snow
crystals. It shows crystal morphology plotted
against temperature and degree of supersaturation, and
shows a sweet spot of conditions where
dendritic classical type snow crystals
form. I wondered if anyone had done the same thing
This diagram from Magono and Lee applied the Nakaya
Diagram to natural snow.
The closest thing is a plot of morphology, referred to as
a morphodrom done by Iwasaki and
Iwasaki. Quartz deposition on the prismatic faces
increases to the right, and the relative development of
the two sets of pyramidal (rhombohedral) faces up and
Fields are defined by the presence of specific faces and
their relations. It is important to consider that a
crystal is defined by its slowest-growing faces.
For example the relative growth rate of the prismatic
m faces increases going to the
right they grow themselves out of existence.
Here are his isotherm lines. Im not clear
what he based them on, but if he is correct, then we
could say that the earlier stages where we find the
longer prismatic crystals are generally cooler, and the
later stages of quartz deposition and overgrowths; the
scepters and the bipyramidal crystals from the Sevier
Shale, took place at higher temperatures. A
path showing change in morphology over time
can be defined. The problem is that a fluid
inclusion analysis for a quartz crystal at Douglas Dam
reported by Edwin Roedder showed much lower
temperatures. Unfortunately, he did not say whether
they were looking at an early prismatic crystal or a late
bipyramidal crystal. The interpretation is questionable,
particularly since bipyramidal crystals are found in
concretions in northern Ohio, far from any orogenic
disturbance. Also, conodont alteration
indices for these rocks do not suggest anything nearly
Time line, with crystal morphologies, showing possible
paragenetic timing. The actual truth is probably
more complex, as the crystals at Douglas dam show
evidence of a stage of reverse scepter or
long prismatic over stubby prismatic, growth. Also,
crystal formation is likely concentrated nearer the
period just before, during, and after the Alleghenian
What further stories can they tell?
Regrowth: Douglas Dam in
particular shows evidence of regrowth after
cracking. This suggests that the crystals were
present before the end of the orogenic event. I
have not found Sevier Shale crystals showing evidence of
cracking and regrowth.
Transport: In most places, most
crystals are completely unworn. hat suggests that
they come from the nearby bedrock, and have not been
moved about by water. But in many places you find
crystals, some of them ARE water-worn. This is somewhat
of a mystery, especially since I have never encountered a
water-worn crystal in areas where they are not otherwise
found very nearby. Its like they show
evidence of being transported without being moved long
distances. One exception is Diamond Creek across
the river from White Pine, where all the crystals show
some degree or erosion and tumbling. Here they
occur in stream gravels in and adjacent to the creek
where its course is exposed in the lake bed. The
creeks watershed is entirely in the Sevier
Shale. The large crystal fragment shown above is
from Diamond Creek.
Abrasion: Crystals found at Muddy Creek
show eroded faces, but it doesnt have the same look
on the edges and points as Diamond Creek specimens.
They may have been in the dirt and soil for several
millennia, where animals stepped on them from time to
Milling: At Douglas Dam, small
concentrations of water-worn crystals are found in the
residual clay, that are very similar to the local unworn
specimens, complete with black inclusions. How
could it be that crystals that appear transported are
concentrated so near similar fresh ones? An
explanation might involve some kind of in-place milling
action in a restricted cavity cause by groundwater
flow. In the zinc mines they have found fragments
of zinc ore in vugs that are rounded and eroded, and that
is the explanation given for those occurrences.
Herkimer type crystals
and other varieties of quartz
growing into open space:
Do Herkimers fit into a
Classification diagram, more of a doodle. The
horizontal axis is whether the quartz is
cryptocrystalline or macrocrystaline, and the vertical
axis is whether the quartz tends to cover all surfaces or
just grow on existing quartz.
quartz refers to when quartz crystals cover all
surfaces, even where the crystals have grown large
through competition, as in a Brazilian amethyst
geode. In drusy quartz, crystals start out small
and numerous, and grow competitively so that there are
fewer larger crystals. Doubly terminated crystals
Where change from one variety to another is observed,
paths can be drawn.
In some agate nodules, deposition goes back and forth
between drusy quartz and agate.
Brazilian agate nodule showing deposition going back and
forth from chalcedonic agate to drusy quartz.
Similar features are observed elsewhere, including in
Detail of above. There are at least four distinct
layers of drusy quartz.
Not found in the Eastern United States, chalcedony roses
occur in silicic volcanic rocks of the West. Quartz
flowers come from locations in Brazil and
elsewhere. For examples search for
quartz flower or amethyst flower
For one of the finest from the Carnegie Museum of
Natural History website. See
One specimen also had anhydrite, suggesting that the
sulfate ion may have a role controlling the morphology of
quartz. For photo see
http://www.Johnbetts-fineminerals.com. One internet
photo, showed a thunderegg that had white chalcedony,
overlying a few quartz crystals on the wall, followed by
a white quartz flower in the center, again suggesting
alternation from chalcedonic to quartz-flower favoring
Examples of chalcedony roses, from Tertiary-age silicic
volcanic rocks in Clark County, Nevada.
Chalcedony roses can be simple,
Or complex forms.
Chalcedony rose transitioning to crystalline growth.
Another instance of fluctuating depositional mode.
This specimen shows a line of chalcedony-covered quartz
crystals (slightly darker gray) growing along the lip.
Internet photo, showing change from chalcedonic to
quartz-flower favoring conditions.
The so-called "cactus quartz" is a case where
larger quartz crystals become covered with finer drusy
Conditions changing in the other direction yield some of
the finest specimens. Some gorgeous examples, with large
late-stage amethyst growing uponwhite drusy crystals can
be found at
Nevertheless, the specific conditions responsible for
these changes remain elusive.