Tennessee Field Diamonds
A treasure 485 million years in the
Crystals near Walland
Contorted and sheared Sevier Shale.
The crystals found in deformed Sevier
Shale at Walland are an exception to the more general
presence of bipyramidal crystals in that formation. They
are found in a road exposure just below the Great Smoky
Fault. These crystals are small, but some are very
Folds in bedded shale and siltstone. This locality is
often visited by geology students for its good examples
Deformed rock contains vugs with crystals in extension
Extension cracks result from a ductility contrast between
the more silty layers and the clay-rich layers. One
might use the analogy of layers of peanut butter cookie
separated by layers of peanut butter. If you
sit on it, the peanut butter may be forced into the
cracks formed as the more brittle cookie cracks and the
entire mass spreads out.
Siltstone and deformed beds of shale.
Closer view of the sheared shale. The elongated
gray surface to left of center is a calcite-lined
extension crack surface with several small quartz
The crack surfaces in this photograph are somewhat dirty,
but calcite-lined and crystal bearing.
Close-up showing calcite-lined extension cracks with
quartz crystals. The objects in the very center are
dew drops trapped in a spider web however. The next
photograph looks down into the small crack to the upper
Deformed layers with crystals growing in extension
Each of the shiny objects in this extension crack is a
Vug in extension crack.
Quartz crystals in vug, small specimen.
reveals some interesting features
Skyscraper tapered form.
Yellow material in inclusion is probably a petroleum or
Phantoms defined by the dark inclusions indicate they
were separate crystals, not in contact at that time.
Inclusion bubbles. See below.
Another view with ultraviolet illumination. Blue
fluorescence indicates petroleum.
Unique terminations occur on some crystals from this
locality. This crystal shows that they developed as
late stage overgrowths.
Detail looking down ath the termination. These are
the pi and omega faces, obtuse rhombohedral
terminations that are generally rare in quartz.
Specimen with unusual faces growing from extension crack
surface. This specimen has a well-developed obtuse
Details of above crystal with pi and omega
What if anything can we say about these crystals?
First, it seems that the ones in the Knox Group
formations show greater variety. Theres
always both elongate prismatic crystals and
bipyramidal crystals, and everything in
between. Transitional forms or
scepters, with 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
section we will see if any conclusions can be drawn.