Many agates have well-defined layers in what has been called “fortification banding.” Such agates have a concentric series of round lines or bands that resemble the circular walls of ancient castles and forts. Hence the term “fortification banding.” In many of these agates, each band has a diff erent and distinct color that makes them beautiful to behold.
But some such agates are truly special, even if they lack color. Take a fortification agate withespecially tight bands that have been cut in half and then polished on a flat lap and hold it in your hand under bright sunlight. When you twist and turn it, you may notice a strong “shadow” eff ect. As you turn the agate from one angle to another, a shadow within the diff erent bands moves and bends across the face of the stone.