black, rugged lava called Malpais (Spanish for 'bad land') there
are caves, just under the surface, containing eternal ice, even in
midsummer, when surface temperatures rise over 40°C. The
scene in this tiny slice of agate, fiery red chalcedony on the out·
side and small quartz crystals on the inside, is an allegory to
this whim of nature. Actual size 45 x 60 mm.
This polished slice of a geode nicely
shows the gradational change from amorphous silica (chalcedony) precipitated in the outer rings, to crystalline silica
(quartz) filling the interior. Actual width 55 mm.
U
G As outlined in the introduction to this chapter, the
interior filling of geodes can be, partly or fully, a precipitate of
chalcedony from aqueous solutions that flow into cavities in
volcanic rock. The solution often percolates into the cavity
through one little duct. That event is here illustrated in graceful
curves. Actual size 30 x 48 mm.
r
A Two thin slices of agate, cut from different geodes and placed on top of each other, create this illusory
water hole. Actual size 40 x 50 mm.
o
Opposite halves of an Australian geode,
the size of a small melon, are suggestively portrayed against a
marine background, evoking a scene that can often be seen in
the seas around that continent. This geode is formed in a void
inside a volcanic rock called rhyolite, the same way as described
for the first few pictures in this chapter.
UI: The forms taken on by chalcedony inside geodes are wondrous and so are the colors, but... , not always. If
I,he colors fall short, well, there are worse things that can hap·
pen.After all, by immersing geode slabs in color baths, they can
be given the fanciest colors you wish. Which is what accounts
for the wondrous blue in this specimen. Actual diameter 70
mm.
G
Geodino is the term used in South America for
small geodes, which often show fine detail, with small and very
clear quartz crystals in the hollow core. This one is about the
size of a walnut.
A CRYSTAL WAlHAll I-l The split and polished surface
61
of a geode, barely the size of a fist, calls to mind the setting of
an ancient Germanic saga. Actual size 60 x 90 mm.
S P
NSeptarian nodules are more or less similar to geodes. They are round claystone concretions with an
interior structure that is star shaped or formed by haphazardly
radiating cracks filled with another mineral, often crystalline
calcite. The cracked structure comes about by the drying up
and shrinkage of the clay concretion and the cracks and voids
are later filled up with calcite. Cut in half and polished, these
nodules display intriguing configurations, like the chap valiantly
marching ahead in this specimen. Actual diameter 120 mm.
AUS
lIAThis type of geode is called a 'thunderegg'. The
colors of the chalcedony filling it are distinctive of the land·
scape and rocks of the Australian outback. No wonder that the
aboriginal people of Australia picked these colors for their
national flag. Actual diameter 120 mm.
are caves, just under the surface, containing eternal ice, even in
midsummer, when surface temperatures rise over 40°C. The
scene in this tiny slice of agate, fiery red chalcedony on the out·
side and small quartz crystals on the inside, is an allegory to
this whim of nature. Actual size 45 x 60 mm.
This polished slice of a geode nicely
shows the gradational change from amorphous silica (chalcedony) precipitated in the outer rings, to crystalline silica
(quartz) filling the interior. Actual width 55 mm.
U
G As outlined in the introduction to this chapter, the
interior filling of geodes can be, partly or fully, a precipitate of
chalcedony from aqueous solutions that flow into cavities in
volcanic rock. The solution often percolates into the cavity
through one little duct. That event is here illustrated in graceful
curves. Actual size 30 x 48 mm.
r
A Two thin slices of agate, cut from different geodes and placed on top of each other, create this illusory
water hole. Actual size 40 x 50 mm.
o
Opposite halves of an Australian geode,
the size of a small melon, are suggestively portrayed against a
marine background, evoking a scene that can often be seen in
the seas around that continent. This geode is formed in a void
inside a volcanic rock called rhyolite, the same way as described
for the first few pictures in this chapter.
UI: The forms taken on by chalcedony inside geodes are wondrous and so are the colors, but... , not always. If
I,he colors fall short, well, there are worse things that can hap·
pen.After all, by immersing geode slabs in color baths, they can
be given the fanciest colors you wish. Which is what accounts
for the wondrous blue in this specimen. Actual diameter 70
mm.
G
Geodino is the term used in South America for
small geodes, which often show fine detail, with small and very
clear quartz crystals in the hollow core. This one is about the
size of a walnut.
A CRYSTAL WAlHAll I-l The split and polished surface
61
of a geode, barely the size of a fist, calls to mind the setting of
an ancient Germanic saga. Actual size 60 x 90 mm.
S P
NSeptarian nodules are more or less similar to geodes. They are round claystone concretions with an
interior structure that is star shaped or formed by haphazardly
radiating cracks filled with another mineral, often crystalline
calcite. The cracked structure comes about by the drying up
and shrinkage of the clay concretion and the cracks and voids
are later filled up with calcite. Cut in half and polished, these
nodules display intriguing configurations, like the chap valiantly
marching ahead in this specimen. Actual diameter 120 mm.
AUS
lIAThis type of geode is called a 'thunderegg'. The
colors of the chalcedony filling it are distinctive of the land·
scape and rocks of the Australian outback. No wonder that the
aboriginal people of Australia picked these colors for their
national flag. Actual diameter 120 mm.