In 1969, the Lake Superior Agate was designated by the Minnesota State Legislature as the official state gemstone. Its wide distribution and iron-rich bands of color reflect the state gemstone's geologic history in Minnesota.
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The Lake Superior Agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred about a billion years ago. The stone's predominant red color comes from iron, the major industrial mineral. The widely distributed agate reveals the impact of glacial movement across Minnesota a mere 10,000 to 15,000 years ago. More than a billion years ago, the North American continent began to split apart into two separate continents. This catastrophic event, spurred by molten rock moving deep within the earth, poured out massive, iron-rich lava flows. These flows now are exposed along the north and south shores of Lake Superior. The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough. The trough eventually became the basin of Lake Superior and the lava flows became the birthplace of Lake Superior agates. Water vapor and carbon dioxide became trapped within the solidified flows in the form of millions of bubbles, called gas pockets or vesicles. Later, groundwater carrying ferric iron, quartz, and other dissolved minerals passed through the trapped gas vesicles.
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The Lake Superior Agate differs from other agates found around the world in its rich red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. The most common type of Lake Superior Agate is the Fortification Agate with its eye-catching banding patterns. Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name Fortification Agate. Probably the most popular Lake Superior Agate is also one of the rarest. The highly treasured Eye Agate has perfectly round bands or "eyes" dotting the surface of the stone.
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