Fortification Agates Agates can take on a wide array of appearances and patterns, but the classic, concentric, band-within-a-band structure most associated with them is known as a fortification pattern, named for its similarity to a fort or castle's walls when viewed from above. These are the "truest" agates, formed under ideal conditions that allowed their banding to properly develop. As such, they also tend to be the most sought after variety of agates, especially examples with colorful and highly contrasting banding. Fortification agates have ring-like bands by definition, but not all specimens exhibit this trait. This is usually a result of a less-than symmetrical pattern but also can simply be a result of weathering. Much of the Pacific Coast saw some glacial activity in the past ice ages, and the immense sheets of ice broke and wore down even the toughest rocks and minerals, including agates. Today, most agates found on beaches are not whole, but merely pieces of larger ones. As such, fortification patterns are often incomplete or obscured.

Fortification agates also frequently have a center cavity, called a geode, as a result of incomplete formation. Similarly, they can also have a center filled with larger, coarser quartz crystals, thought to be a result of there not being enough available silica during formation to develop chalcedony. Color: Varies greatly; usually gray to brown, yellow, orange, or red Structure: Ring-like bands that get smaller as they near the center of the agate, sometimes with a quartz center Rarity: Fortification agates are the most common variety Where to look: You'll find fortification agates virtually anywhere along the Pacific Coast; Oregon's coast is particularly lucrative.