Answer:
Oregon's State rock, the "thunderegg," may be the best known gem
material from Oregon. Thundereggs were not, as believed by some
people, ejected from volcanoes, but formed in very soft and
friable volcanic ash beds. Solutions containing silica permeated
the cinders until favorable points for chalcedony deposition
were achieved. Aggregations of chalcedony were deposited, but
before the material could fully solidify the center of the
concretion split apart, possibly because of shrinkage,
permitting the later introduction of additional materials. The
resulting star-shaped centers of chalcedony may be in the form
of agate, jasper, or in some cases different varieties of opal.
Thundereggs are used in a number of ways. One of the most common
uses is to simply saw the thunderegg into two pieces, polish the
sawed face of each half, and use it as a display or decorative
piece; bookends are also made in this fashion. Also, the
thundereggs are sawed into slabs from which calibrated and
freeform cabochons are cut. Additionally, at least one firm in
the United States is manufacturing gem spheres from thundereggs.
