Lake Superior agates have many characteristics that make them unique, including their colors, shapes, and size:

•  Color

Lake Superior agates are known for their rich red, orange, and yellow colors, which are caused by iron oxidation. The amount of iron and the level of oxidation during formation determine the colors within and between the bands. Some agates can also be brown, and some sections may be so iron-rich that they lose their translucency. 

• Shape

Lake Superior agates are usually shaped like irregular spheres, and can have a pock-marked surface. They can also have conchoidal fractures, which give them a more angular, irregular shape.

•  Size

Lake Superior agates can range in size from about the size of a pea to over 20 pounds in weight. Most are small, averaging about 1 cm in diameter, but some can be much larger. 

• Other characteristics

Lake Superior agates are made of quartz and are often translucent. They can have a dull, waxy luster, and bright bands that are usually not visible from the outside of the unpolished rock. When looking for Lake Superior agates, you can also look for evidence of entrance and escape channels, which may indicate where silica-rich water entered the cavity and pressure escaped during the agate precipitation process. 

The agates were formed about a billion years ago during lava eruptions in Minnesota, when air bubbles trapped in the lava cooled and water seeped into the holes, depositing minerals in layers.   

Small agates are often polished and used in jewelry, while larger agates are sometimes cut into thin slabs to display their colored bands. The rarest Lake Superior agates are those that weigh 2 pounds or more and have perfect shape, color, and banding quality, but only about one in 10,000 agates fit this description.

The agates' predominant red color comes from iron, a major industrial mineral in Minnesota. The agates also have rich bands of orange, tan, and yellow caused by different minerals.

The agates were dispersed by glaciers during the last Ice Age, which moved them to other places and abraded, fractured, and polished them. The agates' wide distribution reveals the impact of the glaciers' movement across Minnesota 10,000 to 15,000 years ago.Once the glaciers retreated, the Earth's crust started to rebound (rise). The result is that the north side of Lake Superior keeps rising by about one foot (30-50cm) per century compared to lands along the southern edge of the Great Lakes. This process is called "isostatic rebound" by geoscientists.