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Still Iffy on banding- Is this a Blue Banded Agate?

+1 vote

Once again- You all must be sick of me by now!


asked Oct 25, 2015 by Dizzard (830 points)

1 Answer

+2 votes
Yes, nice stone, it is a geode with agate inside, a type of banding called fortication because the banding looks like separate walls of a fort repeated over and over. A lapidary has cut this one and polished the surface. Any clue where it is from?
answered Oct 25, 2015 by Weasel (58,940 points)
Let me see if I can explain the banding. It is believed agates were formed when there was volcanic activity and a much different topography and climate. Holes in the cooling lava were formed by pockets of air. The rock was somewhat porous material. Silica is what quartz is made of. The most common rock. Well, at the time there was a lot of silica in a much wetter environment. The silica was dissolved in the water and it seeped into the holes in the rock in different ways, then dried out leaving the minerals that were dissolved in the water (in solution) inside the air chamber in a layer. After occuring thousands of times each separate layer or band was formed, that is why agates are mostly banded. The term banded agate is in reference to the banding most agates have. Different minerals in solution with the silica cause different colors, most of the red banding in lake superior agates is from the iron, blue maybe aluminum. Many from the Houghton area have copper banding, those are rare and copper inclusions. But there are tubular agates, too. Anyhow, quartz is a macrocrystalinear form of silica rock and chalcedony and agate are microcrystalinear forms of silica. If you rub a piece of quartz you can feel the grain. If you rub an agate it will feel smooth like wax because the grain is much smaller. Agate is very hard. The host rock gets abraded away (weathering) exposing the agate banding. In Lake Superior, the waves and sand expose the agate without cutting them.
I'm sorry I have no clue where this one is from. Thank you so much for the explanation. Every bit get closer to being able to do this on my own-My Goal stone is not Amazonite upon closer inspection - the war is back on! Did you happen to see the weird stone I published yesterday- in a million years I will never get this one. As for the agate I am going to look for the other half would make a nice set!

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