Geodes are more or less spherical nodules of chalcedony with
an interior that may be hollow or (partially) filled with quartz.
Quartz and chalcedony are, respectively, the crystalline and
amorphous (non-crystalline) variety of silica (Si02). The chalcedony in geodes may also be referred to as agate, but that
term is meant to indicate a chalcedony of semi-precious quality, discussed in the next chapter. Flintstone, chert and silex are
other denominations for chalcedony.
The origin of geodes is explained by the precipitation of silica
(or more rarely some other mineral) from circulating subsurface waters, inside hollow spaces in volcanic rocks. These hollow spaces are usually the congealed voids that have remained
from gas bubbles that originally occurred in the hot lava. The
precipitation of silica from percolating water or vapor is a slow
process, responding differentially to various circumstances, like
concentration and temperature. By these processes imaginative and colorful scenery emerges, with combinations of banded chalcedony, crystalline quartz, the occasional rosettes of
hematite (iron OXide) and dendrites of pyrolusite (manganese
oxide). Sawing geodes in halves or in slices, and polishing the
cut surfaces, brings out all this imagery.
A place known - already since ancient times - for its geodes,
agates and other beautiful minerals is Idar-Oberstein in West
Germany. Famous makers of jewelry and objects in stone, for
example Faberge, came here to find their materials. Although
most of the indigenous resources have now been depleted,
Idar-Oberstein has carried on its fame as the worlds 'epicenter'
for precious and semi-precious stones. Numerous workshops
where stones are fashioned into jewelry and trinkets as well as
many shops and several museums make it a lively business center and a collector's paradise.
GEODE
theory for the origin of these Iythophyses, very complex and
difficult to comprehend for the outsider. Fortunately, as with
many other things of beauty in this country, these precious little gems can be more than adequately enjoyed without complicated explanations.
ALY
The portentous scenery inside this rhyolitic
geode is no bigger than a thumbnail. See above for a more
detailed description.
U
00
This rooster, in a rhyolitic
geode (see above) from Australia, stands face to face with his
alter ego, from France. Is this coincidental? Or is it a conspiracy,
something beyond the understanding of plain mortals? Actual
width 35 mm.
LThis proud rooster stands in a small,
rhyolitic geode (see above) from the Esterel region in southern
France. For some inscrutable reason the precipitation of chalcedony, inside rhyolitic geodes, often takes on the outlines of
poultry. Like the crowing rooster on the facing page, flown in all
the way from Australia.Actual diameter 40 mm.
LThis tasteful agate, the size of a plum, is also from
the Esterel Mountains on the French Mediterrannee. The surrounding green rock is rhyolitic lava. The void inside has been
filled up with clear chalcedony, which is full of little red rosettes
of hematite, an iron oxide. If this were a dinner course prepared by Bocuse, it would be called Escargots aI'hematite dans
une croute de rhyolite.
believe it. In western New Mexico, in the inaccessible terrain of
Aslice of agate, from a Brazilian geode not
larger than a peach, is depicted here in transmitted light. The
German craftsman who cut and polished this slice saw nothing
less in it than the Urknall.
thin section, photographed
with transmitted, polarized light (see page 112), shows the
complex details of precipitation and crystallization inside a
geode, with banded rings of chalcedony on the periphery and a
core of crystalline quartz.Actual size 15 x 24 mm.
U
Waferthin laminae of intensely red
hematite surround a mysterious, black hole of quartz crystals.
On the polished face of an Esterel geode the size of a golf ball,
cut in half. Geodes of this type, of which we see a few more in
the next plates, come from the Esterel region on the French
Mediterranean coast and they are quite characteristic. The surrounding greenish-grey host rock is rhyolite, a volcanic rock. It
encloses a core of chalcedony, stained by the vivid reds of
hematite. To the French geologists who studied these geodes
they are also very special. So special that they did not simply
call them geodes but rather assigned them a more dignified
name,'Lithophyses'.Also, these scientists worked out a deviant
60
L
You have to see it to
an interior that may be hollow or (partially) filled with quartz.
Quartz and chalcedony are, respectively, the crystalline and
amorphous (non-crystalline) variety of silica (Si02). The chalcedony in geodes may also be referred to as agate, but that
term is meant to indicate a chalcedony of semi-precious quality, discussed in the next chapter. Flintstone, chert and silex are
other denominations for chalcedony.
The origin of geodes is explained by the precipitation of silica
(or more rarely some other mineral) from circulating subsurface waters, inside hollow spaces in volcanic rocks. These hollow spaces are usually the congealed voids that have remained
from gas bubbles that originally occurred in the hot lava. The
precipitation of silica from percolating water or vapor is a slow
process, responding differentially to various circumstances, like
concentration and temperature. By these processes imaginative and colorful scenery emerges, with combinations of banded chalcedony, crystalline quartz, the occasional rosettes of
hematite (iron OXide) and dendrites of pyrolusite (manganese
oxide). Sawing geodes in halves or in slices, and polishing the
cut surfaces, brings out all this imagery.
A place known - already since ancient times - for its geodes,
agates and other beautiful minerals is Idar-Oberstein in West
Germany. Famous makers of jewelry and objects in stone, for
example Faberge, came here to find their materials. Although
most of the indigenous resources have now been depleted,
Idar-Oberstein has carried on its fame as the worlds 'epicenter'
for precious and semi-precious stones. Numerous workshops
where stones are fashioned into jewelry and trinkets as well as
many shops and several museums make it a lively business center and a collector's paradise.
GEODE
theory for the origin of these Iythophyses, very complex and
difficult to comprehend for the outsider. Fortunately, as with
many other things of beauty in this country, these precious little gems can be more than adequately enjoyed without complicated explanations.
ALY
The portentous scenery inside this rhyolitic
geode is no bigger than a thumbnail. See above for a more
detailed description.
U
00
This rooster, in a rhyolitic
geode (see above) from Australia, stands face to face with his
alter ego, from France. Is this coincidental? Or is it a conspiracy,
something beyond the understanding of plain mortals? Actual
width 35 mm.
LThis proud rooster stands in a small,
rhyolitic geode (see above) from the Esterel region in southern
France. For some inscrutable reason the precipitation of chalcedony, inside rhyolitic geodes, often takes on the outlines of
poultry. Like the crowing rooster on the facing page, flown in all
the way from Australia.Actual diameter 40 mm.
LThis tasteful agate, the size of a plum, is also from
the Esterel Mountains on the French Mediterrannee. The surrounding green rock is rhyolitic lava. The void inside has been
filled up with clear chalcedony, which is full of little red rosettes
of hematite, an iron oxide. If this were a dinner course prepared by Bocuse, it would be called Escargots aI'hematite dans
une croute de rhyolite.
believe it. In western New Mexico, in the inaccessible terrain of
Aslice of agate, from a Brazilian geode not
larger than a peach, is depicted here in transmitted light. The
German craftsman who cut and polished this slice saw nothing
less in it than the Urknall.
thin section, photographed
with transmitted, polarized light (see page 112), shows the
complex details of precipitation and crystallization inside a
geode, with banded rings of chalcedony on the periphery and a
core of crystalline quartz.Actual size 15 x 24 mm.
U
Waferthin laminae of intensely red
hematite surround a mysterious, black hole of quartz crystals.
On the polished face of an Esterel geode the size of a golf ball,
cut in half. Geodes of this type, of which we see a few more in
the next plates, come from the Esterel region on the French
Mediterranean coast and they are quite characteristic. The surrounding greenish-grey host rock is rhyolite, a volcanic rock. It
encloses a core of chalcedony, stained by the vivid reds of
hematite. To the French geologists who studied these geodes
they are also very special. So special that they did not simply
call them geodes but rather assigned them a more dignified
name,'Lithophyses'.Also, these scientists worked out a deviant
60
L
You have to see it to