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Geodes are sphere-shaped stones, usually at least
partially hollow, and often lined inside with sparkling mineral crystals
or concentric layers of minerals
as river beds erode, geodes tumble into streams.
Geodes are rocks that are plain on the outside but can have beautiful
crystals on the inside. In the Greek language, geode means "shape of the
earth", and geodes are round like earth or oblong like an egg. They can
be a couple inches or several feet in size.
Geodes begin as bubbles in volcanic rock or as animal burrows, tree
roots or mud balls in sedimentary rock. Over time, the outer shell of
the spherical shape hardens, and water containing silica precipitation
forms on the inside walls of the hollow cavity within the geode. The
silica precipitation can contain any variety of dissolved minerals, the
most common being quartz, but amethyst and calcite are also found.
Over a period of thousands of years, layers of silica cool, forming
crystals of different minerals within the cavity. Different types of
silica cool at varying temperatures, thus creating layers of different
types of mineral crystals.
- volcanic rocks that contain numerous gas cavities
filled with secondary minerals such as zeolites, calcite, chalcedony
or quartz
- filled cavities are called amygdules
Geodes are found throughout the world, but the most
concentrated areas are located in the deserts. Volcanic ash beds, or regions
containing limestone, are common geode locations.
There are many easily accessible geode collecting sites in the western
United States, including California, Arizona, Utah and Nevada. The state of
Iowa is also has geodes, in fact, the geode is their state rock.
- cryptocrystalline quartz and much chert
- material agate is made of
- has a great array of colors: blue, gray, black,
off-white, purple (decomposition of iron-bearing minerals)
- in Hall’s Gap geodes
- sulfide mineralization occurs in chalcedony geode
Each geode is unique in composition and can only be truly
discovered when cracked open or cut with a rock saw. The size and
formation of crystals and different shades of color within the crystals
make each geode special. The rough exterior of the geode gives no
indication of the secrets held within its core. The anticipation never
fades for those who curiously collect buckets full of round geodes and
eagerly expose the secrets of
each individual sphere-shaped rock. The most prized contain rare
amethyst crystals or black calcites.
- hollow, globular bodies
- sub spherical shape
- clay film between geode wall & the enclosing
limestone matrix which is like a skin or crust
- an outer chalcedony layer
- an interior drusy lining of inward projecting
crystals
- evidence of expansion or growth
- slow deposition of groundwater causes crystals to
grow
- estimated it takes 240 million years to form
- geodes form either in sedimentary rock or in
basalts/andesites
- hollows appeared in sedimentary rocks either
because of animal burrows, tree roots or mud balls
- word "geode" derived from Latin meaning
"earthlike", rounded shape
- found in USA: Montana, Indiana, Iowa, Illinois,
Pennsylvania (Lancaster Co. goethite geodes), Tennessee (Loretto),
Oklahoma (Comanche County etc.), Michigan (Houghton), Dugway geodes,
Colorado (Chaffee County ,Marshall Pass), Hauser Geode Beds,
California
- amethyst geodes from Artigas, Uruguay or near
Lajeado, Rio Grande Do Sul, Brazil
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