agates

Chalcedony quartz with colorful stripes and interesting inclusions.

Chinese Rain Flower Agates

Chinese rain flower agates are found in Nanijing, China in the bed of the Yangtze River.  They can also be called Yuhashi or Yuhua stones.  The river polishes them smooth, and people often display them in a small bowl of water to bring out the colors.  They look very similar to Lake Superior agates, and like Lake Superior agates, they are named after where they came from.

Landscape Agates

A landscape agate is any agate with inclusions that appear to be a landscape scene.  This usually includes plumes or dendrites, because they look so much like trees. I was unable to find the original sources for some of these cabochons, but I think the beautiful photos speak for themselves.  Which one is your favorite?

Enhydro Agates

A blue-gray polished agate with circular swirls.  You can't see it, but there is water inside.

Photo by Verity Woolf, from collection of Jo Woolf.  Retrieved from: http://the-hazel-tree.com/2014/01/06/enhydro-agate-a-secret-water-chamber/

Sometimes, agates will have a little bubble of water trapped inside.  This is called an enhydro.  The water inside is millions of years old!  The water can be seen if you shine a light on the agate from behind, or heard if you shake it.  Eventually, the water will work its way out and evaporate via small capillaries, but the hollow part will still remain.  It is also possible that water could enter the agate through the same capillaries.  Enhydros can be found in watery places where agates are found, such as Brazil, Indonesia, and Oregon.  Photos don’t show this very well, but on the link in the photo, the owner has some videos that show off the water inside.

For more information about enhydros, go here: http://www.rocksinmyheadtoo.com/Enhydros.htm

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.

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