The Lake Superior agate is a fortification agate that fills cavities in basalt flows that formed over a billion years ago in the Lake Superior region. Over time, silica-rich groundwaters filled these cavities with agate and crystalline quartz. Most of them have been completely infilled and are more properly called a “nodule.” However, some still retain a cavity that is often lined with crystalline quartz. The agate within them is typically reddish brown, red, and orangish red in color. These colors are caused by trace amounts of iron that was incorporated in the agate. Today they are found along beaches, in stream channels, in the soil of farmers’ fields and in glacial till.