Natural Science Forum / Earth Science / Mineralogy / July 2004

I found a softball size rock in my backyard that had the look of quartz to
it. It is dark root beer in color and will scratch a file but not be
scratched by a file indicating a hardness greater than 6.5. I had a friend
slice off the end with a rock saw. It is not translucent near the edges. My
first thought was a hunk of Jasper, but noticed very small inclusions mixed
in a random pattern in a light gray. Being from an area high in igneous rock
(within 20 miles of Mt. Rainier in Washington State) a second thought would
be a highly silicified Basalt or Rhyolite. Most of these rocks that I have
seen while having a fine grain, do not resemble massive quartz. I would
think any Igneious rock somewhere between Obsidian and Rhyolite would be
less than 6.5 in hardness. I will find a piece of Quartz and try the scratch
test again but expect them to be the same hardness. Anyone have any ideas to
further identify this rock? I brought it to my local rock shop and their
guess after some hesitation was Rhyolite.

Thanks,
Rick