The Agatelady: Adventures and Events: Jequitinhonha Brazilian River Agates

First of all, I would like to apologize to all those who asked me for information about the rare Brazilian river agates after I showed them my "plate tectonic" agate. I told several people that I had included the Jequitinhonha Agate as a mineral of the month on the www.agatelady.com web page. I checked this morning and found that I had not. However, pm July 22nd of 2011 I posted a blog update with information about thee agates.

Then I received an email from someone who purchased one of the river agates requesting additional information. I decided to do a little more digging. I did several Internet searches and was not able to find a scientific article written specifically about these incredible river agates. However, I did find some references that seem to verify that the story I have been telling people about these agates is in fact true.

First, here are some photos of the Jequitinhonha agates. This first photo shows the specimen I purchased two years ago from the guy who acquired the collection. Last year I purchased several and sold them out right away. This summer when I met with my friend, Brian, who had the collection -- I decided to purchase the rest of the Jequitinhonha agate collection. After all, Brian wasn't even telling the story to possible customers. He did tell people they were naturally polished, but that is as far as Brian went in his explanation. Well, I guess I cannot help myself -- I love these rocks. I now have around 40 of these agates for sale in the museum and will bring several to all of the remaining shows. Check my schedule for both art shows as well as gem and mineral shows by scrolling down the home page at www.agatelady.com.

When people first see these agates that strut their highly polished surfaces, everyone assumes that they have been coated with varnish. That is not the case. These incredible agates became free from their basaltic matrix and ended up in the rivers of southeastern Brazil. In the sands of some of the rivers, including the Jequitinhonha River located in Minas Gerais, Brazil, there is diamond dust as well as diamonds. In fact, diamonds have been mined in Brazil since 1725. Over thousands of years, the river agates became naturally polished by the diamond dust resident in the sands of the rivers.

But where did the diamonds come from? Scientists do not know for sure. They do agree that the majority of diamonds are associated with Kimberlite, which is a type of volcanic rock known for sometimes containing diamonds. This rock is named after the town of Kimberley in South Africa, where the discovery of an 83.5-carat (16.7 g) diamond in 1871 spawned a diamond rush.

Kimberlite occurs in the Earth's crust in vertical structures known as kimberlite pipes. The consensus on kimberlites is that they are formed deep within the mantle at depths between 93 and 280 miles (150 - 450 km). Scientists think that these kimberlite pipes contain enriched exotic minerals that are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting that creates high enough temperatures and pressures to produce kimberlites prone to hosting diamond xenocrysts.

A diagram of a kimberlite pipe is below, as is a picture of the top of one of these geologic structures that has filled in with water.

There are two significant facts regarding the presence of diamonds and diamond dust in Brazil. First of all, kimberlite tubes have never been found in Brazil. So if there is no source to explain why diamonds are in Brazil, where did they come from? The answer actually can be explained by the presence of an unusual type of diamond in both Brazil and Africa that is different than all the other diamonds found elsewhere in the world. These are "black diamonds." Pictures of some of these gemstones are below.

Diamonds with black as the primary dominant color are unique. Black diamonds, also known as carbon diamonds, owe their color to minute inclusions and random clustering throughout the stone rather than trace elements like nitrogen, boron and hydrogen that produce color in most diamonds. The black fine-grained diamond aggregate have a unique luster that seems to emanate from sheer darkness because a pure black diamond has almost complete absorption of light.

Black diamonds are found only in Brazil and the Central African Republic. Conventional diamonds formed deep in the earth's crust and were brought to the surface through kimberlite pipes. But where did black diamonds come from? And why are there only two places you can find them that are on opposite sides of the Atlantic Ocean? The exact formation process for these black diamonds and why they are so different is yet to be discovered, but scientists have explained the reason why there are only two deposits now located thousands of miles apart. South America and Africa were once connected landmasses. Today the deposits of black diamonds, and perhaps the regular kimberlite pipe diamonds, are on two separate continents because the landmass that was once connected has now split into separate landmasses that have drifted apart. The Atlantic Ocean lies between the diamond fields in South America and Africa. But when the continents of South America and Africa were last connected, perhaps the rivers of what are now Brazil drained the diamond fields of Africa.

During the earth's geologic history there have been several times that all the earth's landmasses have been connected as a supercontinent. The last time was 200 million years ago when South America and Africa were connected as part of the Pangea supercontinent. A map of what scientists think the supercontinent looked like is below.

The approximate location of the Jequitinhonha River in the Minas Gerais province of Brazil is shown below.

Here is a map showing the location of the Minas Gerais province in Brazil.

And here is a map showing Brazil and the location of the African diamond fields. The red dot shows the mouth of the Jequitinhonha River.

Below is another map of what scientists think the connection looked like between South America and Africa. When they studied the landforms, they noticed that the land forms matched between the two continents. The curved lines in the map below represent structural trends. These are the directions of major geologic structures. Upon examination of the landforms on either side of the Atlantic Ocean, scientists have concluded that the structural trends appear to be continuous across the continental boundaries, as if they had once been connected. Not only that, but the ages of the rocks match, as do localities of mineral resources such as tin, iron, and diamond deposits.

So it seems that the Jequitinhonha Brazilian River agates were naturally polished by the African diamonds and diamond dust deposited in the sands of the Brazilian river between 200 and 300 million years ago when the continents were last connected.