Refiner of Gold Creations

"Thundereggs" or Geodes


This page is a collection of tidbits on the geodes or "Thundereggs" and soon will include links to other geode sites.

A geode is "a hollow, more or less globular body, up to 30cm or more in diameter, found in certain limestones and volcanic rocks, and rarely in shales. Significant features include a thin outer layer of dense chalcedony; partial filling by inward-projecting crystals, generally quartz or calcite, but sometimes barite or celestite; and evidence of growth by expansion. Unlike a druse, a geoge is separable from the rock in which it occurs and its crystals are not of the same minerals as those of the enclosing rock." (A druse is "an irregularly cavity or opening in a vein or rock having its interior surfaces or walls encrusted with small projecting crystals, usually small minerals as those of the enclosing rock, and sometimes filled with water.") (Bates)

Geological theory states that:

Thundereggs were formed during periods of activity in Oregon and Washington about 30 million years ago. Glassy volcanic material crystallized to form ball-shaped 'sperulites.' The central cavaties of these balls were forced open by gases, thus forming the outer egg-shaped shell of the thunderegg. The cavity was later filled in stages with gel to form the patterend or banded central agate core.

The Indian Legend:

At times, the neighboring snow-capped peaks of Mt. Hood and Mt. Jefferson of the Cascade Mountain Range would become angry with one another. The skies would darken, thunder could be heard and the gods would hurl round masses of rock at one another. These rocks were the Thundereggs robbed from the nests of the thunderbirds.

Commonly used as decorations, geodes are attractive and may have partially filled or wholly filled cavaties. They are formed when minerals grow along the margins of the cavity. Common in limestone beds or found among other rocks, most geodes are 30cm (15") or less in diameter. The geode's outermost layer is a thin and sometimes discontinous layer of agate, which is a variety of color-banded and compact microcrystalline quartz (SiO2). Banding within the geode is parallel to the walls of the cavity. In partially filled geodes, the innermost layer of inward-pointing crystals are of quartz or calcite (CaCO3) or of various sulfates as barite (BaSO4) and celestite (SrSO4). All of these inner minerals were precipitated from a solution. (Monroe)

  • Bates, Robert L. and Julia A. Jackson, eds. Dictionary of Geoliogical Terms, 3rd ed. Prepared by the American Geological Institute. Garden City, New York: Anchor Press/Doubleday. 1984.
  • Monroe, James S. and Reed Wicander. The Changing Earth: Exploring Geology and Evolution. 2nd ed. Belmont, California: West/Wadsworth, an International Thomson Publishing Company. 1994.
  • Thunderegg: "Unique Volcanic Rock". Chehalis, Washington: Pony Butte Products.

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