An Interview with Virginia Gillerman
Virginia Gillerman is the research geologist for the Idaho Geological
Survey. She spoke with the OUTDOOR IDAHO crew in May of 1996.
Q. WHY ARE THERE SO MANY GEMS IN IDAHO?
Gillerman: Idaho had a traumatic past, which helps account for
our diversity of gems.
There are a wide variety of geologic terrains in the
state. That's why we get something like the garnet, which
forms at great depths, and something like opal, which
forms on the surface.
As you go back in time, about 100 million years ago,
there is a collision boundary between the Pacific plate
and the North American plate; this collision zone goes
along Highway 55 & 95, north to south, through western
We have rocks that are as old as two billion years and
as young as two thousand years; and that helps create the
geological and chemical conditions to form a number of
different minerals at different times in the state's
WHAT ARE GEMS?
Gillerman: Gems are actually a subset of minerals, which are
common building blocks of all of Idaho's rocks.
Quartz is one of the common minerals in Idaho, but only a
few forms of quartz are valuable as gemstones. The
difference has to do with the color of the stone, the
clarity, the transparency, whether it's free from
fractures, and the hardness.
HOW ARE GEMS FORMED AND WHERE DO YOU FIND THEM?
Gillerman: Gems occur in just about any rock type; and they
form through a number of ways. Often it has to do with the interaction
of hot water with the rocks.
The simplest example is petrified wood. It starts out
as an ordinary tree trunk; then it is solisified through
the interaction of hot water going through the log and
replacing it with silica.
Most of the Owyhee desert is ordinary rhyolite; but in
a few places either along fractures or along large round
cavities created by gas in the magma, late stage gas and
water came through and deposited silica in volcanic rocks.
Sometimes they are called thunder eggs.
You find jasper and agate geode fillings in some of them.
It takes a lot of looking. Sometimes they only release
their crystal shapes when cut in half.
Metamorphic rocks are found predominantly in northern
Idaho; and this rock is host to the star garnet, Idaho's
official state gem. The star garnet is found only in Idaho
Garnet is a common metamorphic mineral. The rock may have
started out as ordinary sediment, a shale, and through the
action of geologic time at great depth and temperature,
the minerals in shale re-composed themselves as a new
mineral denser in form.
I think hot water is often over looked in forming gems; it
is particularly important in forming opal and jasper. Hot
water has dissolved constituents, silica being one of the
major ones, along with other elements -- like iron,
fluorine, berrylium -- that give gems their color.
These fluids are often found along faults; fluid
circulation is driven by heat: geo-thermal heat is a good
way to look at it. Forty million years ago it was the heat
of cooling magma in the Sawtooths.
You need fluid in the tight rocks to carry silica,
aluminum, iron into one small space where it can grow into
a crystal. So fluid is often an overlooked component of
the process of gem formation.
ANY WORDS OF ADVICE TO NEW ROCKHOUNDS?
Gillerman: There are a few things to look out for, aside from
the snakes in the rocks. Be prepared for a lot of hard work, because
gems are so rare. Gems are locatable in terms of mining claims,
so make sure you're not on someone's property.
Be prepared to swing a pick ax. You have to be
lucky, too; there's no beter quality than luck when
looking for gems! It's a good hobby for anyone from seven
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