Courtesy Mark RyanA special group of rock hounds gathered over the weekend in the Twin Cities to celebrate and give praise to agates, those special gemstones found in just about every country of the world. “A Celebration of Agates” was held July 26-29, 2012 at the Lindbergh Center at Hopkins High School in Minnetonka, MN.
Courtesy Mark RyanThe weekend event was hosted by the Minnesota Mineral Club and included presentations, book signings, banquets, plenty of vendors, and lots of agates – piles of them, on tables, in buckets, inside display cases, all there for the public to enjoy.
Each year at the Minnesota State Fair, I man a booth for the Geological Society of Minnesota. It’s great talking geology with visitors, answering their rock and geology questions. But by far the most frequent questions and discussions are about agates - specifically, those found in the Lake Superior region. It’s not surprising that the Lake Superior agate is Minnesota's state gem.
Courtesy Mark RyanThese beautiful gemstones formed inside the empty spaces (vesicles) created by gas bubbles trapped in lava flows that poured out during the Mid-continental Rift (MCR) episode about a billion years ago and cooled into fine-grained basalts. Later, when the basalts were buried deep under sediments, ground water or hydrothermal activity flowing through the sequence deposited minerals (mainly silicon dioxide) inside the empty spaces layer by layer - and from the inside out - forming amygdules. Trace minerals and impurities can add color to each layer, creating bands of different color. In the Lake Superior area, the rocks became exposed again, and the basalts began to weather leaving the harder amygdules to fall out. Glaciers transported agates from the Lake Superior region and deposited them with tons of other rocks forming gravel pits to the south of the lake. Some of the best Lake Superior agates can be found around the Twin Cities.
Courtesy Mark RyanPersonally, I never got into collecting them, I’ve always been mostly into fossils, but I have to admit, some of those agates I saw at the show were stunningly beautiful pieces of natural art. There’s something very attractive about a collection of Lake Superior agates covering a tabletop or laid out inside a display case. But Minnesota’s state rock wasn’t the only agate on display. Participants from many states and several countries including India, Australia, Germany, and South America brought their collections and knowledge to share with other agate lovers.
Courtesy Mark RyanI usually don’t buy rocks, fossils or minerals – I like to find my own – but after I watched one vendor use some sort of strange bolt-cutting tool to break open several large geodes visitors had selected, I had to try my own hand at it.
Buckets of geodes lined a nearby table. The largest were as big as a softball but picked out a modest, three-inch diameter "Mexican Coconut” mined in Chihuahua, Mexico.
Courtesy Mark RyanThese geodes formed in a similar manner as most agates: a hollow space left by gas bubbles in a cooling lava flow allowed for minerals to line the interior and crystals to grow as groundwater flows through it. Sometimes agate (chalcedony i.e. silicon dioxide) forms inside, other times it can be any number of iron-oxides, silicates (quartz), or calcite. The “Coconut” geodes formed in a 44 million year old ash-flow tuff that over time eroded into whitish clay. The geodes are mined from the clay 200 feet below the surface. Geode is a general term for a rock with a hollow space inside. Sometimes geodes can contain agate material such as chalcedony or jasper, and sometimes agates can be considered geodes (if they have a hollow space), but the two terms aren’t always interchangeable.
According to the very helpful vendor, the trick to picking the most primo geode was to find one the feels lighter than others of comparable size. It would logically follow that it has a larger hollow space and therefore possibly more crystal growth inside.
Courtesy Mark RyanSo, I picked out a good one, and the vendor took it and tightened what looked like a large bike chain around the stone, then applied pressure on the cutter handle. After a sharp crack of sound, the geode broke open and fell into his hand in two equal halves. Inside each was a beautiful blue, milky lining of quartz dotted with dark, rod-like crystals of goethite, a hydrated iron oxide.
The event also featured a black-light tent for viewing florescent minerals, videos about agates, hourly drawings, on-going silent auctions, geological tools and lapidary supplies for sale, a ton of agates, and a whole flock of agate-lovers more than happy to show their favorite finds to the droves of rock hounds who came to see them.
Courtesy Mark RyanI think my favorites were the three stunning grapefruit-sized agates that grace the cover of a recent book titled Agates of Lake Superior written by Dan and Bob Lynch from Two Harbors, Minnesota. They had all three beauties on display. Two were found in the Two Harbors area, and the other in a gravel pit near Forest Lake, MN.