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geode

geode (jē´ōd), hollow, globular rock nodule ranging in diameter from 1 to 12 in. (2.54–30.5 cm) or more. Most geodes are partly filled with mineral matter; they have a thin layer of chalcedony ( "wavy" quartz) covering an inner lining of inward-projecting crystals. These spectacular crystals, generally quartz or, less often, calcite, make geodes highly prized by collectors. Geodes are formed in a cavity such as might be found inside a fossil shell buried in sediment. At the beginning, this cavity is probably filled with a concentrated salt solution. The first step in the creation of a geode is the formation along the inner cavity wall of a layer of gelatinous silica, which will eventually be transformed into the chalcedony layer. As the water surrounding the layer becomes less salty, osmosis induces migration of fluids into the cavity. This results in a buildup of pressure, causing the cavity to expand until the water inside and outside is equally salty. When the silica gel dehydrates, crystallizes to form chalcedony, and cracks, mineral-bearing water enters to slowly deposit the inward-projecting crystals. A geode measuring 26 ft (8 m) long and 6 ft (1.8 m) across was found in an old silver mine near Almería, NE Spain, in 2000. See also concretion.

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geode

ge·ode / ˈjēōd/ • n. a small cavity in rock lined with crystals or other mineral matter. ∎  a rock containing such a cavity. DERIVATIVES: ge·od·ic / jēˈädik/ adj.

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geode

geode A hollow, rounded body, which has a lining of mineral crystals pointing inward, e.g. quartz or calcite. The crystals grow into the cavity unimpeded and form perfect crystals which are frequently collected and valued for their beauty.

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geode

geode. Piece of rock having a druse, or cavity lined with crystals, or a rounded hollow iron-stone nodule used e.g. in rock-rash facings.

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geode

geode XVII. — L. geōdēs — Gr. geṓdēs earthy. f. earth.

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geode

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