RHYOLITE

(Fr- rhyolite; Ger- Rhyolith; Nor- rhyolitt/liparitt; Rus- [= liparite] )

RHYOLITE

A. Rhyolite. Two lower specimens are silicified rhyolites ("Wonderstone") from an unknown locality (thumb and figers give scale). Anna Jonas Stose collection. The top specimen is silicified breccia, not rhyolite, the dark fragments of which are epidote-rich algal-like masses (see Dietrich and Chyi, 1995). R.V. Dietrich collection. (© photo by Dick Dietrich)

B. Rhyolite. Silicified rhyolite (diameter - 6.5 cm) from unknown locality. This illustration is a computer-generated mock up showing how the banded rhyolite (lower right piece in "A") could be used to fashion a book-match pendant. (© photo by Dick Dietrich)

DESCRIPTION: Rhyolite is the aphanitic equivalent of granite -- i.e., rhyolite has the same general composition as granite but the individual grains of rhyolite are so small they cannot be distinguished by the naked eye or even with the aid of a handlens. Many rhyolites, however, contain macroscopic phenocrysts, and some rhyolites also have natural glass as a constituent.
    Color (i.e., the overall color of rhyolites per se and of the ground masses of rhyolite porphyries) -- light gray, pink, yellowish, cream, mauve, tan to medium brown, light to brick-red; mottled or banded patterns are relatively common.
    H. (effective hardness) > 5, typically 6 - 7
    S.G. ~ 2.65
    Miscellany -Rhyolites that exhibit certain kinds of flow banding and/or contain spherulites are the ones used most frequently as gemrocks.   Many rhyolites are porphyritic with quartz and salmoncolored K-feldspar plus or minus white to off-white sodium-rich plagioclase as the most common phenocrysts; identification of the minerals that occur as phenocrysts in aphanite porphyries is the method used by most geologists to name handspecimens of chiefly aphanitic rocks in the field.   In the absense of phenocrysts, the overall color is often utilized, but noted with prduent disclaimers.

OTHER NAMES: ... Rhyolite, with the ellipsis replaced by a geographic designator plus or minus an adjective describing its color or some quality such as its being porphyritic is the way many chiefly rhyolite masses are named to in the geologic literature and on geologic maps. Two examples are the Mule Mountain Rhyolite of southwestern New Mexico and the Balaklala Rhyolite of Klamath Mountains province of California.

A few names given rhyolites that have been used as gemrocks follow:

Birdseye - name given to some spherulitic rhyolites.
Elixerite - local name for rhyolite wonderstone from the vicinity of Truth or Consequences, Sierra County, New Mexico.
Hickoryite - rhyolite, some of which resembles rhyolite wonderstone, from near Rodeo, State of Durango, Mexico .
Liparite - synonym applied to rhyolite, particularly in the past in Europe, especially in Germany and Russia.
Mushroom rhyolite - cabochons, beads, pendants have been marketed widely as as mushroom rhyolite, which is said to have been metamorphosed. It is said to come from Arizona. I have seen only photographs of the so-named material, which indicate that the pattern exhibited by the polished surfaces resemble mushrooms, but provide nothing that I could see that would indicate it to be rhyolite. Correspondance about it has furnished no pertinent data.
Wonderstone (sometimes referred to with geographical or other adjectives such as Nevada Wonderstone) - a thinly banded rock, commonly -- albeit with exceptions -- rhyolite. The banding involves one or more of the following colors: off-white, reddish brown, orangish yellow, yellow-brown, purplish brown, medium brown, and dark gray to nearly black. Unfortunately, this term is also applied rather frequently in the marketplace to banded jasper and banded sandstone that have been used as gemrocks.

USES: Jewelry -- especially that including fairly large stones; diverse ornaments.

OCCURRENCES: Most rhyolite occurs as lava flows. Several origins have been suggested to account for the banding of rhyolites designated wonderstone. Any serious consideration of those suggestions is far too long and involved include here; for those who wish to delve further into this subject, one possibly fruitfuI start would be to look up Liesegang banding in geological and chemical data bases.

NOTEWORTHY LOCALITY: Much so-called gem quality rhyolite comes from Australia. A well-known United States locality is near Truth and Consequences, Sierra County, New Mexico.

REMARKS: The name rhyolite, from the Greek word ρύαξ (stream) -- ergo, streaming rock -- was apparently given in allusion to the flow banding that is so characteristic of much of this rock. This name was first applied in the literature in 1860 by Baron Ferdinand von Richthofen, grandfather of the (in)famous World War I pilot, Manfred von Richthofen, the "Red Baron." Today, rhyolite is used rather widely as the first term of binomial rock names -- e.g., rhyolite porphyry, rhyolite breccia and rhyolite tuff -- as well as a freestanding noun for rocks of this composition be they banded or not. The already mentioned synonym liparite has also had widespread use, especially in Europe. It was Introduced in 1861 by Professor Justus Roth of the University of Berlin for occurrences in the Lipari Islands, off Italy. The fact that the term rhyolite was introduced a year before liparite gives it priority so it is generally thought the term liparite should be abandoned. However, a slight quirk so far as abandoning the term liparite has been mentioned: Theophrastus used the term Λιπαραοϛ (liparaios) for stones from the Lipari Islands that were very likely at least a variety of this rock. (see Johannsen, 1932, v.II, p.265)

The term rhyolite always comes to my mind whenever I think of confusion related to names of rocks. This is so because i have found it virtually impossible not to think of the following descriptio: “Rainforest Jasper is a beautiful, opaque pastel semi-precious gemstone, displaying hues of green with occasional clear spots. Referred to as Rhyolite by Geologists, Rainforest Jasper is really a Granite that has cooled relatively quickly." (www.luckygemstones.com ) -- Wow!!!

All sorts of problems and ambiguities attend usage of the term Wonderstone. Indeed, diverse applications of the term comprise a nomenclature nightmare. Not only has the name been applied to some so-called scenic sandstone -- see SANDSTONE entry -- but also to a jasperized calcareous breccia from the Eagle Peak area of Sierra County, New Mexico. In addition, I suspect that some similar jasperoid of the same general region, which has been given the local name Candy rock (Rouse, 1963.), would be considered a wonderstone, at least by connoisseurs of rock candy, eh(?).

The cliff dwellings that are one of the areas of particular interest in Bandelier National Monument in north central New Mexico are in rhyolite tuff. These cavelike dwellings, some of which have been dated back to the mid 12th century, were occupied by the Anasazi (ancestral Puiebloans). Some of them are only slightly altered natural cavities; others were enlarged by carving into this relatively soft rock.

SIMULANTS: None that I have seen or seen described.

REFERENCES: No general reference.

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