Last updated
A thunderegg from the Black Rock Desert in Nevada that has been cut in half Black rock desert thunderegg.JPG
A thunderegg from the Black Rock Desert in Nevada that has been cut in half

A thunderegg (or thunder egg) is a nodule-like rock, similar to a filled geode, that is formed within rhyolitic volcanic ash layers. [1] Thundereggs are rough spheres, most about the size of a baseball—though they can range from a little more than a centimeter (one half inch) to over a meter (three feet) across. They usually contain centres of chalcedony which may have been fractured followed by deposition of agate, jasper or opal, [1] either uniquely or in combination. Also frequently encountered are quartz and gypsum crystals, as well as various other mineral growths and inclusions. Thundereggs usually look like ordinary rocks on the outside, but slicing them in half and polishing them may reveal intricate patterns and colours. A characteristic feature of thundereggs is that (like other agates) the individual beds they come from can vary in appearance, though they can maintain a certain specific identity within them.


Thunderegg is not synonymous with either geode or agate. A geode is a simple term for a rock with a hollow in it, often with crystal formation/growth. A thunderegg on the other hand is a specific geological structure. A thunderegg may be referred to as a geode if it has a hollow in it, but not all geodes are thundereggs because there are many different ways for a hollow to form. Similarly, a thunderegg is just one of the forms that agate can assume.


Thundereggs are found globally where conditions are optimal. In the US, Oregon is one of the most famous thunderegg locations. Germany is also an important center for thunderegg agates (especially sites like St Egidien and Gehlberg). Other places known for thundereggs include Ethiopia, [2] Poland, [3] Romania, Turkey, Mexico, Argentina, [4] Canada, Mount Hay [5] and Tamborine Mountain (Australia), [6] and the Esterel massif (France). [7]


Thundereggs are found in flows of rhyolite lava. They form in the lava from the action of water percolating through the porous rock carrying silica in solution. The deposits lined and filled the cavity, first with a darker matrix material, then an inner core of agate or chalcedony. The various colors come from differences in the minerals found in the soil and rock that the water has moved through. [8]

State rock designation

On March 30, 1965, the thunderegg was designated as the Oregon state rock by a joint resolution of the Oregon Legislative Assembly. [9] [10] [11] While thundereggs can be collected all over Oregon, the largest deposits are found in Crook, Jefferson, Malheur, Wasco and Wheeler counties. [12] The world's largest thunderegg, a 1.75 ton specimen, is housed by the Rice Northwest Museum of Rocks and Minerals in Hillsboro, Oregon. [13]


Native American legend reportedly considers the rocks to be the eggs of the thunderbirds which occupied Mount Hood and Mount Jefferson. Thunder Spirits on the mountains hurled the "eggs" at each other. [11]


See also

Related Research Articles

<span class="mw-page-title-main">Agate</span> Rock consisting of cryptocrystalline silica alternating with microgranular quartz

Agate is the banded variety of chalcedony, which comes in a wide variety of colors. Agates are primarily formed within volcanic and metamorphic rocks. The ornamental use of agate was common in Ancient Greece, in assorted jewelry and in the seal stones of Greek warriors, while bead necklaces with pierced and polished agate date back to the 3rd millennium BCE in the Indus Valley civilisation.

<span class="mw-page-title-main">Chalcedony</span> Microcrystalline varieties of silica, may contain moganite as well

Chalcedony ( kal-SED-ə-nee, or KAL-sə-doh-nee) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO2 (silicon dioxide).

<span class="mw-page-title-main">Jasper</span> Chalcedony variety colored by iron oxide

Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to iron(III) inclusions. Jasper breaks with a smooth surface and is used for ornamentation or as a gemstone. It can be highly polished and is used for items such as vases, seals, and snuff boxes. The specific gravity of jasper is typically 2.5 to 2.9. Jaspillite is a banded-iron-formation rock that often has distinctive bands of jasper.

<span class="mw-page-title-main">Chert</span> Hard, fine-grained sedimentary rock composed of cryptocrystalline silica

Chert is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood.

<span class="mw-page-title-main">Onyx</span> Banded variety

Onyx is the parallel-banded variety of chalcedony, an oxide mineral. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands; agate has curved bands while onyx has parallel bands. The colors of its bands range from black to almost every color. Specimens of onyx commonly contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".

<span class="mw-page-title-main">Volcanic rock</span> Rock formed from lava erupted from a volcano

Volcanic rock is a rock formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks.

<span class="mw-page-title-main">Geode</span> Hollow formation inside a rock

A geode is a geological secondary formation within sedimentary and volcanic rocks. Geodes are hollow, vaguely spherical rocks, in which masses of mineral matter are secluded. The crystals are formed by the filling of vesicles in volcanic and subvolcanic rocks by minerals deposited from hydrothermal fluids; or by the dissolution of syn-genetic concretions and partial filling by the same or other minerals precipitated from water, groundwater, or hydrothermal fluids.

<span class="mw-page-title-main">Vug</span> Small to medium-sized cavity inside rock

A vug, vugh, or vugg is a small- to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity are partially filled by quartz, calcite, and other secondary minerals. Open spaces within breccias formed by an ancient collapse are another important source of vugs.

<span class="mw-page-title-main">Lithophysa</span> Felsic volcanic rock

A lithophysa is a felsic volcanic rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticular. They vary in size from very small up to twelve feet in diameter depending on the age of the magma chamber. These rocks are usually found within obsidian or rhyolite lava flows. Lavas low in feldspar minerals may produce a version known as snowflake obsidian.

<span class="mw-page-title-main">Moss agate</span> Agate variety

Moss agate is a semi-precious gemstone formed from silicon dioxide. It is a form of chalcedony which includes minerals of a green color embedded in the stone, forming filaments and other patterns suggestive of moss. The field is a clear or milky-white quartz, and the included minerals are mainly oxides of manganese or iron. It is not a true form of agate, because it does not have concentric banding.

<span class="mw-page-title-main">Quartz-porphyry</span> Type of volcanic rock containing large porphyritic crystals of quartz

Quartz-porphyry, in layman's terms, is a type of volcanic (igneous) rock containing large porphyritic crystals of quartz. These rocks are classified as hemi-crystalline acid rocks.

<span class="mw-page-title-main">Lake Superior agate</span>

The Lake Superior agate is a type of agate stained by iron and found on the shores of Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota, Wisconsin, Nebraska, Iowa, Kansas and Michigan. In 1969 the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.

<span class="mw-page-title-main">Hogg Rock</span> Mountain in the Cascade Range of northern Oregon

Hogg Rock is a tuya volcano and lava dome in the Cascade Range of northern Oregon, United States, located close to Santiam Pass. Produced by magma with an intermediate andesite composition, it has steep slopes and thick glassy margins. Hogg Rock exhibits normal magnetic polarity and is probably about 80,000 years old.

<span class="mw-page-title-main">Chrome chalcedony</span>

Chrome chalcedony is a green variety of the mineral chalcedony, colored by small quantities of chromium. It is most commonly found in Namibia and Zimbabwe, where it is known as Mtorolite, Mtorodite, or Matorolite. It is also marketed using the trade name, Aquaprase.

<span class="mw-page-title-main">Rice Northwest Museum of Rocks and Minerals</span> Historic house in Oregon, United States

The Rice Northwest Museum of Rocks and Minerals is a non-profit museum in Hillsboro, Oregon, United States. Located just north of the Sunset Highway on the northern edge of Hillsboro, the earth science museum is in the Portland metropolitan area. Opened in 1997, the museum's collections date to the 1930s with the museum housed in a home built to display the rock and mineral collections of the museum founders. The ranch-style home is listed on the National Register of Historic Places, the first of its kind listed in Oregon. In 2015 the museum became a Smithsonian Affiliate museum.

<span class="mw-page-title-main">Enhydro agate</span>

Enhydro agates are nodules, agates, or geodes with water trapped inside its cavity. Enhydros are closely related to fluid inclusions, but are composed of chalcedony. The formation of enhydros is still an ongoing process, with specimens dated back to the Eocene Epoch. They are commonly found in areas with volcanic rock.

<span class="mw-page-title-main">Mineral and Lapidary Museum</span>

The Mineral and Lapidary Museum of Henderson County is a non-profit, volunteer-run museum in Hendersonville, North Carolina, United States, founded in 1997 at 400 North Main Street in the middle of the city's Historic District.

<span class="mw-page-title-main">Petersen Rock Garden</span> Rock garden and museum in Central Oregon, U.S.

Petersen Rock Garden, formerly Petersen's Rock Garden and also known as the Petersen Rock Gardens, is a rock garden and museum on 4 acres (1.6 ha), located between the cities of Bend and Redmond in Deschutes County, Oregon, United States. Rasmus Petersen, a Danish immigrant who settled in Central Oregon in the early 1900s, began constructing the garden in 1935 using rocks he found within an 85-mile (137 km) radius of his family home. Petersen constructed detailed miniature castles, churches and other small buildings and monuments from a variety of rock types. He incorporated other design elements such as bridges, water features, and natural landscaping. Petersen worked on the garden until his death in 1952; the garden has remained in his family's care since then. The garden, considered a roadside attraction with novelty architecture, includes roaming peafowl and a museum with a gift shop that sells rocks.


  1. 1 2 "America's Volcanic Past - Oregon". USGS Cascades Volcano Observatory. Archived from the original on 2013-09-05. Retrieved 2007-06-08.
  2. Rix, David. "Thundereggs from Ethiopia". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  3. Rix, David. "Thundereggs from Poland". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  4. Rix, David. "Thundereggs from Argentina". Eibonvale Thunderegg Gallery. Retrieved 19 May 2017.
  5. "Mount Hay". Dig the Tropic: Outback to the Reef. Capricorn Enterprise. Retrieved 19 May 2017.
  6. "Tamborine Mountain". Sydney Morning Herald. The Sydney Morning Herald. 15 August 2008. Retrieved 19 May 2017.
  7. Richard, Jean-Jacques (1 February 2009). "Les Oeufs de Tonnerre". Bijoux et pierres precieuses (in French). Retrieved 19 May 2017.
  8. Thunderegg Oregon State Rock, StateSymbols USA
  9. "Oregon Symbols". SHG Resources. Archived from the original on 2012-02-08. Retrieved 2007-09-17.
  10. Chapter 186 — State Emblems; State Boundary 2017 Oregon Revised Statutes
  11. 1 2 "Rock Hounding". Nature of the Northwest. Archived from the original on 2007-09-28. Retrieved 2007-09-17.
  12. "Oregon Almanac". Oregon Blue Book. Retrieved 2007-09-19.
  13. Christie, Tim (March 26, 2007). "Rock hounds check out goods at 18th annual Gem Faire". The Register-Guard. Eugene, Oregon.