The age of a seashell is dependent on a number of factors, including species of the animal carrying the shell. For bivalve organisms, the age of the shell can be determined by looking at cross-sections of the shell. An alternation in color is thought to represent a year's growth.
Unfortunately, this method of age detection is destructive to the shell. The color of the cross sections is typically denoted by large white bands offset by smaller black bands. There is still some variability in the exact age of the shell, because it can be stunted by changes in weather, but this method is fairly accurate in approximate annual age. A common misconception is that outer rings, visible without cutting the shell, are representative of age, but similar to trees, cross sections must be analyzed to get a better estimate of age.