Lake Superior Agates formed a billion years ago from volcanic activity in the Lake Superior Region. This allowed silified water into pockets of rhyolite and basalt. The granular details of how the agates form in the host rock is still unkown. A great read on agate formation and chrystallography can be found here.

Since their formation, a large portion of the agates have been carried by glaicers and processes of erosion south of Lake Superior. Some have even made it to the very end of the Mississippi River.

Texture and Luster

Agates are made of microcrystalline quartz which appears to be like a waxy, plastic, glass type material.

Fractures

Agate or Microcrystalline Quartz fractures in a particular pattern called Conchoidal Fracturing. The fractures are often circular or curved in nature and are easily identified under magnification.

Translucency

Many of the rocks have light that can pass through them. A common thing to look for is a rock that appears to look like red or orange glass.

Banding

Agates have a unique way of layering that occurs in most of the rocks which forms like the banding of growth rings in a tree.