The Lake Superior agate is noted for its rich red, orange, and yellow coloring. This color scheme is caused
by the oxidation of iron. Iron leached from rocks provided the pigment that gives the gemstone its beautiful array of color.
The concentration of iron and the amount of oxidation determine the color within or between an agate's bands.
The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about
1 cm in diameter. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms.
Very large agates are extremely rare.
A 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.
A subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. Perfectly
straight, parallel bands occur over all or part of these stones. The straight bands were produced by puddles of quartz-rich
solutions that crystallized inside the gas pocket under very low fluid pressure. The parallel nature of the bands also indicates
the agate's position inside the lava flow.
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.
Most of the agates here I collected myself on the beach in McLain State Park, north of Houghton, Mi., and
are of the more common plain carnelian type.
