Images

Agates and Jasper

Jasper and agates are both a type of quartz known as chalcedony. So it makes perfect sense that sometimes you can find jasper and agate melded together, like so. Agates are usually transparent or translucent, and jasper is usually opaque, but this isn’t a hard and fast rule. Sometimes it’s very difficult to tell the difference between agate and jasper, in which case people may call it chalcedony, agate jasper, or jaspagate. It is mostly found in the American Southwest and California, like other agates and chalcedonies. The one pictured was found near the San Andreas Fault.

Thunder Eggs

The thunder egg was declared the official state rock of Oregon in 1965, because there are quite a lot of them there. A thunder egg is a rounded nodule or geode with agate in the center.  Thunder eggs can also contain quartz, chalcedony, crystals, or opal.  The inside parts can be opaque or transparent – there are almost as many possible designs as there are agates.  This unusual thunder egg shown above is from Oregon and has plume inclusions. Some thunder eggs are also geodes but this one is not a geode because it does not have crystal points. A more typical thunder egg would look similar, but with bands or a single color on the inner part.

Think Pink: Think Thulite
A roughly triangular chunk of rock covered with bubblegum-pink splotches. They sparkle slightly against the contrasting blue background.

Photo by David J. Eicher, http://www.mindat.org/photo-378407.html

It’s pretty hard not to notice that October is Breast Cancer Awareness Month.  Everything is awash in pink here, even the fountains.  So, I thought I would highlight an interesting pink mineral.  Thulite is a a pink variety of zoisite, a crystal, and it’s pink because it contains manganese.  Thulite is named after Thule, which was the ancient name for Norway (or sometimes when talking about far-off places in general, like when Virgil wrote about “Ultima Thule”).  It is most often found in Norway, Scandinavia, and sometimes North Carolina.  Susan G. Komen estimates that 232,030 cases of invasive breast cancer will be diagnosed in the U.S. this year, so now that you’ve read this, go do your breast self-exam!

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

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