Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course

 


October, 2006

Meteorites as Gemstones

Although most of us are familiar with meteorites as interesting collector items, far fewer realize that they, along with some related specimens, can be beautiful additions to a gem or jewelry collection.

Much of the essay that follows was written by one of my colleagues at CCSN, Dr. David Batchelor, a specialist in planetary geology. In Astronomy 103: The Solar System, one of the courses David teaches (in person and online through CCSN's Distance Education Department), these topics are developed in greater detail.

Irons, Pallasites, Tektites and Impactites in Jewelry

Most meteorites are pieces of asteroids, which are leftover building blocks of the planets. The more common meteorite types are chemically and geologically primitive, and while their chemistry may offer a fascinating glimpse into the planet-forming process, they are both too fragile and too drab to be of gemological interest (most look like ordinary dull grey rocks). Nevertheless, there are some meteorites and related materials suitable for jewelry.

Irons: A handful of asteroids grew large enough to differentiate; they had enough gravity to pull their iron content away from the rock and down into a core. Iron meteorites, which are really alloys of iron, nickel, and other metals, are fragments of these cores.

[An iron meteorite in a freeform natural shape which might find a good home in a pendant]

Their fascination as jewelry comes primarily from their Widmanstätten patterns, which are seen only after a piece is cut, polished, and etched with 2% nitric acid in alcohol. At the concentrations found in these meteorites, iron and nickel do not mix, but separate into two types of crystals; plates of the low-nickel alloy kamacite grow in octahedral shapes, with the high-nickel taenite alloy filling in the spaces. Specimens from deeper within the parent asteroid have cooled more slowly, and their crystals have grown larger. Much of the lapdiary skill in fashioning these pieces comes from selecting a piece with appropriate crystal size, cutting it to display the pattern to advantage, polishing and etching it to bring out the pattern, and somehow protecting it from rust.

One of the most common of the irons seen in jewelry form are the Gibeon meteorites. This name is applied to those obtained from the strewn field of a fall which occurred in prehistoric times in the region of Gibeon in the Southern African nation of Namibia. These pieces are about 92% iron and nearly 8% nickel and can be dated by radioisotopes to nearly 4 billion years in age.

[This pair of earrings features both sliced and etched, as well as simple tumble polished nuggets of iron meteorites]

[Gibeon meteorite tablet "cabochons" with larger and smaller Widmanstatten patterns. The piece in the 14k gold pendant has been rhodium plated to resist rusting]

[This group has been plated with 24k gold, another attractive way to protect the metal from rusting and corrosion while preserving the crystal patterning]

Pallasites: The most beautiful, and some of the most expensive, of all the meteorites, the 50 or so Pallasites are iron meteorites with silicate rock inclusions, often of amber to pale-green olivine crystals, gemologically known as peridots. The Pallasites all come from the core/mantle boundaries of three different parent asteroids, and are collectively named after Peter Simon Pallas, who described the first known example, found near Krasnoyarsk in Siberia. They can have intact olivine crystals large enough for jewelry use.These specimens are often sliced to give a stained-glass effect, or, rarely, individual peridots are faceted. A find in Argentina in the 1950's yield the Esquel Pallasites which have relatively large and clean gem areas in with stable metallic matrix that can be highly polished. As that find is exhausted the pieces are now passing from collector to collector and increasing in price as they go. The piece seen below measuring about 2" x 2" inches and weighing less than 12 grams at present has a retail price of about $1000.

[This magnificent (and really valuable) slice from the Esquel Pallasite has large "phenocrysts" of peridot interspersed within the metallic matrix. Image courtesy of www.gemfrance.com]

[A sterling silver pendant featuring a polished Esquel Pallasite slice (seen with reflected light to show the metallic sections and with back lighting to reveal the peridot pockets, a pair of tiny gem peridots (1.3 mm each, .07 ct. tw) cut from a Pallasite meteorite.]

This URL will take you to a photo of the largest faceted Pallasite peridot gem known to exist (1.5 ct.): http://www.meteoriteman.com/collection/peridot.htm

Tektites: When meteorites hit the surface at high speeds, the target rock, soil, or sand is melted, and droplets solidify into glassy tektites. They occur in four particular areas on Earth*, three of which are centered around known impact craters.

*(The number of named tektite strewn fields has actually shrunk over the years, as what were previously believed to be separate field were found to be parts of larger fields spreading across multiple continents. The Bediasites from Texas and the Georgiaites from Georgia are now recognized as part of the North American strewnfield from the 34 million year old Chesapeake Bay crater. The Australites, Chinites, and Indochinites are now part of the Australasian strewn field, for which no crater has been found despite a relatively young age of 600,000 years. And 1 millon year old old tektites from Africa and Australia are part of the Ivory Coast strewn field associated with a crater at Lake Boumtwi in Ghana.)

Although there is significant evidence tektites formed from craters on Earth, a handful of prominent researchers cite contradictory evidence, and argue for an origin from craters on the Moon. Some of the prettiest tektites are from the strewn field along Europe's Moldau river, associated with the 15 million year old Ries Crater in Germany. These gems, known as Moldavites, are a translucent to near transparent sagey to olive green, with surfaces apparently shaped by wind as they descended through Earth's atmosphere. Other tektites, depending on their location of origin, and exact composition, range from brown to black in color and from near transparent to opaque.

[Rough Moldavite and Chinese tektite showing, flattened, bubbly, rough surfaces]

[Carved Moldavite, faceted Moldavites in pendant with iron meteorite, Chinese tektite as body of stickpin with South Sea pearl]

Impactites: One of the most mysterious materials in this general group is typified by a material called Libyan Desert Glass. Found in remote areas of the Sahara often in quite large pieces, technically, the Libyan Desert Impact Glass samples aren't tektites, although they are pieces of glass formed by meteorite impact. Tektites specifically show aerodynamic shapes from reentry through the Earth's atmosphere. (Or entry, if you allow for the possibility of a lunar origin.) The more general term "impactites" includes tektites, melt glass, impact breccias, shattercones, or any bit of geologic evidence of impact. Notice in the rough piece shown below the lack of the tektite-like aerodynamic features. The glassy nature of this material is revealed by the presence of gas bubbles, but at least in this piece, they are much smaller and more scattered than is typical of Moldavite or other transparent tektites.

2

[Libyan Desert Glass: rough piece (about 1" x 1.5"), view at 30x of bubbles in interior, a faceted piece]

This URL will take you to a picture and story about a recent archeological finding of a scarab gem, carved from Libyan Desert Glass among King Tut's jewelry: http://news.bbc.co.uk/2/hi/science/nature/5196362.stm

Man-Made "Meteorite" Gemstone

This is a bit of a stretch, but kind of fun to include in this essay. In 1893 a French scientist, Henri Moissan, began studying fragments of meteorites from Arizona's Meteor Crater. Dr. Moissan discovered microscopic crystals of quantities of a new mineral, a form of silicon carbide. In 1905, this mineral was named Moissanite, in his honor. Although opaque, non-gem forms of silicon carbide could be found and/or manufactured, and were used extensively as industial and lapidary abrasives (hardness = 9.5), none of the transparent, single crystal gem type was made until 1995. Recently Charles and Covard Company has patented the production process and is offering the near colorless form of the gem as a diamond simulant.

[Moissanite synthetic silicon carbide diamond simulant]

Value Factors

For the most part, meteorites and related "astro" gems can be had for relatively modest prices considering their beauty and interest. Specimen collectors and those interested in jewelry are now in competition for a smaller and smaller pool of available pieces, so my advice is to buy sooner rather than later.

Gemological Properties:

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Go to: Homepage -- what's new at ACS -- monthly specials and discounts -- "buried treasures" -- BWS/FS jewelry designs -- gem topic of the month -- gem topic archive -- birthstones -- ask Barbara -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- setting these gems -- free gemology course