Thank you for the helpful feedback, Kenton; also to others who
responded as I made notes from all of the postings.
I have found samples from two outcroppings now, about 300 feet apart
in elevation on the mountain with lots of overburden in between.
Certainly these samples are interesting from a geology perspective
but I have little idea if they also have lapidary/jewelry interest so
I'll expand some more.
I am not sure yet if the outcroppings are closer to original
sedimentary or igneous deposits. The geological maps say the overall
formation is Jurassic (the end of the dinosaur age) but they describe
both sedimentary and volcanic rocks in the outcroppings so it is more
complex than a single time of formation. Volcanic andesite carries
much feldspar. Repeated periods of volcanic activity could also
transform the original lavas or the sediments derived from them.
The host rock is coal black but it is highly fractured. Some
geologists think this region has seen a major earthquake about every
500 years. The fracturing seems to leave some rock faces exposed to
weathering and these turn a dark, rusty brown. I have seen a number
Labradorite samples on web sites and those with the lighter
background rock seem to yield more obvious iridescence so the darker
background probably detracts from the aesthetics. It is only on the
weathered faces that the iridescence appears and it is lost when you
make a rough cut with a geologist's hammer into the black host rock.
I don't have the equipment to do slab cuts.
So far my hunch/theory is that the weathered feldpar is transformed
by later periods of nearby volcanic activity which glaze these
surfaces and produce all the colours of the rainbow but (as sampled
so far) only on the weathered surfaces so whatever the chemical
transformation on those surfaces it seems necessary for the
colouring. Red, orange, yellow, green, blue, indigo and violet are
all apparent so far. Some samples display a few colours and others
display a different part of the spectrum but all are in the deposit.
The greens and blues are especially impressive because I have never
seen such vivid colours before in my prospecting trips. They look
like marine enamel paint has been dobbed on the rock. That is, they
are deep or saturated colours. Green is as common as the other
colours except blue which seems to be the rarest. The uni-colour
"spotches" are typically 1-2 mm in diameter. My hunch/theory is that
the black host rock is andesite-like with a high feldpar content. The
original lava was eroded and became a high feldspar content
sedimentary rock. Subsequent volcanism then glazed the weathered
surfaces which produced the rainbow colours. One of our rockhound
group looked at the rainbow surface under a loop of about 20x and
it looked "bubbled". It does. The corners are rounded which fits in
with the notion of melting and glazing.
Under a loop of about 20x, the colour is quite stunning. I
thoroughly enjoy observing these surfaces in good sunlight and under
the loop but as I said above, I have no idea how jewellers might work
the stones, if at all. The dark, rusty-brown background detracts from
the rainbow effects and the "splotches" are small. But who knows what
lies deeper inside the deposit as there is an abundance of this
material near the surface.
Iridescence seems to have to do with thin films or layers as we read
at http://www.webref.org/geology/i/iridescence.htm and in this case
they seem to only coat the weathered surfaces along natural fault
lines. As we read at http://en.wikipedia.org/wiki/Iridescence the
word comes from Gk "iris" = rainbow. And the complete rainbow is
found on these rock faces.
I have a nice sample here which shows all the colours of the rainbow
across a surface of about 15 mm x 5 mm. The entire piece of host rock
is about 100 grams and we would be happy to mail it to someone who is
experienced in working with labradorite, on a no cost/no obligation
basis. You can look at it and throw it out or send it on to someone
else if you need a second opinion etc. Let me know off-list.