Agate Month

Iris Agates

A round, thin, flat slice of agate with rainbow-colored concentric circles that makes it look very much like a compact disc.

Specimen from the National Mineral Collection, photo by Chip Clark. http://geogallery.si.edu/index.php/en/1177807/agate

Iris agates are very finely banded (15-30 bands per millimeter) iridescent agates that I like to call CD agates.  In order to see if an agate has the iris effect, you must bring it home, cut off a thin slice, and hold it up to the light.  The slices also have to be cut perpendicular to the banding to see the rainbows.  The rainbow effect occurs because the thin layers of agate split up the visible light into many beams of different wavelengths, which correspond to different colors.  This is also known as a diffraction grating, and it’s also the reason why CDs have rainbow patterns.

If you would like a technical explanation of how iris agates act as diffraction gratings read this: http://www.minsocam.org/msa/collectors_corner/arc/iris.htm

Eye Agates

An eye agate is an agate that is marked with one or more concentric circles, which look like eyes. The circles are usually a complementary color to the rest of the agate. The eyes are actually little spheres, possibly caused by silicification around a seed crystal of some other material located at the edge of the agate while the rest of the agate is forming. They just look circular because you see their cross-section. Sometimes two eyes will merge, creating a figure-8 inside the stone, as seen in the close-up photo.  Eye agates are pretty rare, but they seem to occur more often in agates from Lake Superior and carnelian agates from Botswana.

Lake Superior Agates

Lake Superior Agates are a type of agate found near Lake Superior.  They were carried across Minnesota by glaciers 10,000 years ago and are so common there that they have been Minnesota’s official state gemstone since 1969.  Wisconsin and Michigan have some, too.  Lake Superior agates are known for their red color which comes from iron oxide in the surrounding area.  Most Lake Superior agates are banded agates, but a few are eye agates, some are “waterwashed” agates (called so because they have been naturally polished by the water on the beach, like sea glass), and, rarest of all, some of them weigh over 2 pounds.

I should also point out that if you ever go looking for agates, the ones in the field won’t be as colorful and they are easy to mistake for other rocks like granite or basalt.  Most of the pictures of agates are of nice pretty polished slabs or cabochons, and that tends to give people unrealistic expectations.  That’s why the photo shows Lake Superior agates before and after tumbling.

The Minnesota Department of Natural Resources was invaluable in writing this post.  More information here: http://www.dnr.state.mn.us/education/geology/digging/agate.html

Argentina Condor Agates

These are colorful banded agates found in San Rafael in Mendoza Province, Argentina.  They were discovered by the Argentinean actor and rockhound Luis de los Santos in 1992. He named them Condor agates because of the eponymous birds native to the mountains.  Sometimes knowing where a rock was found is just as important as what it’s made of.

Turritella Agate

A dark brown, almost black square of rock, cut flat, completely covered with images of tan, tubular shells pointing in various directions.

Photo by Stephanie Reed

Turritella agate is a fossiliferous agate that contains lots of snails who died, sank to the bottom of a lake, and became silicified. When it was first named, people thought the fossils were marine snails from the Turritella genus. The shells are actually from the freshwater snail Elimia tenera, but the Turritella name was too popular already and it stuck. They are found in the Green River Formation in southwestern Wyoming, northeastern Utah, and northwestern Colorado, because that’s where the snails used to live (they are extinct). This specimen is from the personal collection of Show-Me Rockhounds member David Reed.  Fun fact: Elimia tenera snails became fossilized in materials other than agate, such as limestone, so there is also turritella limestone. It’s not quite as pretty in my opinion because the light background doesn’t contrast with the shells like agate does.

Fire Agate

A triangular brown stone with the front face cut off and polished revealing iridescent shimmering red, orange, yellow, and a little bit of green colors inside.

Photo of a polished fire agate found in Aguascalientes, Mexico by Rob Lavinsky/irocks.com retrieved from http://www.mindat.org/photo-180670.html

Fire agates contain iridescent (rainbow) colors that resemble fire.  The iridescence comes from the diffraction of light between the fire agate’s alternating layers of silica and iron oxide.  This is called the Schiller effect.  You can also see the Schiller effect in labradorite and mother-of-pearl (hmmm, could there be a post about labradorite coming soon?).  It’s difficult to photograph iridescence, so any photos you see are probably even more beautiful in person.  Fire agates are rare, but may be found in Arizona, northern Mexico, and other parts of the Southwest.  Like other agates, they rate 6.5-7 on the Mohs scale and are gorgeous when polished.

Dendritic Agate

Dendritic agate has inclusions that look like thin, two-dimensional tree-like growths.  They are usually black or dark brown, especially if they are found in Montana.  Often dendrites form, between flat “waterline” bands of agate.  Dendrites may also occur in limestone, talc, sandstone, beryl, corundum, and other minerals.  Dendritic and moss agates both look amazing as cabochons, because they look like little pressed flowers or landscape scenes.  The specimen pictured is from Del Norte, Colorado.
Source: The Gemrock, 7/2014

Sagenitic Agate

Sagenitic agate is any agate having acicular or needle-like mineral growths inside.  These hair-like filaments are often arranged in fans or starbursts.  The inclusions come in a wide array of colors.  Sagenite has been found in over 250 different agate deposits worldwide, a little in most agate fields, probably less that 5% of the available agate in most fields.  This agate is from California and contains mostly yellow sagenite but also a few plumes; can you spot them?

Source: The Gemrock, 7/2014

Update: It looks like somebody at IGAMS (and the CMS Tumbler, The Clackamette Gem, and The Glacial Drifter, all of whom were listed as sources) might be a fan of Pat McMahan’s website Agates With Inclusions, as the text from their article on agates that I read is very similar to this article on sagenite and plume on his website.  So, thank you for the inspiration, Mr. McMahan!

Plume Agates

Light red translucent agate with red and orange puffy plumes appearing to float inside it.

From the collection of Don Volkman, photographed by Karl Volkman. http://www.mindat.org/photo-152268.html

Plume agates have fluffy inclusions which often appear to be soft and have depth.  Sometimes, plume agate inclusions resemble feathers, plants, or flowers.  Possible colors include red, orange, yellow, pink, and white.  Plume agates are most commonly found in Oregon’s Graveyard Point, Idaho, Colorado, West Texas, and Mexico.  The agate pictured is from Texas.

Source: The Gemrock, 7/2014
From the collection of Don Volkman, photographed by Karl Volkman. http://www.mindat.org/photo-152268.html