The Middle Cambrian is what we call the time period from 2500 to 1600 million years ago. It is important to the economic history of Minnesota, and to the development of this nation as an industrial giant. Why? Well, sometime during this era, the atmosphere changed from one rich in carbon dioxide to an atmosphere rich in oxygen. This happened because of the development of colonies of green marine plants. The oxygen released iron in the underlying rocks and deposited it as hematite (iron ore), which makes up the iron ranges in Minnesota. This happened world wide. The iron ranges on other continents have been determined, by geochronologists, to be the same age. Other interesting geological events happened in Minnesota around this time, including some mountain building, but this one was probably the most spectacular.
I opened up the small tumblers and found some nice Minnesota agates and other agate, jasper and quartz material. Also, I ordered tumbled rocks, to get more variety and to start making sure I have reasonably priced examples of all the birthstones. I started with January, and ordered garnet and rose quartz. Then put the rocks that needed more polish back in the tripoli powder. I still don't have road signs but I'm carrying on as if I did :)