By Naturalist William Hudson
If you are one of those folks who still enjoy walking along a beach and collecting shells, driftwood, and other treasures, you may have found a small, pure white, roughly circular little stone about the size of a penny that resembles a flattened pearl or piece of milky glass. If you look closely and find ridges on one side that resemble an "L" or a "C", you may have found a fascinating artifact known as a lucky stone or fish pearl.
On the beaches of the Great Lakes these are likely the ear stone or "otolith" of a common freshwater drum called a sheepshead or gaspergou. The name gaspergou is a corruption of the French "Casse burgau" or mussel breaker. Lucky stones have a very interesting history. Because Native Americans kept lucky stones as charms or tokens of good luck, they are often found in archeological sites going back thousands of years. It is also said that Great Lakes fishermen and sailors kept lucky stones in their pockets to keep them safe from storms, to give them an edge in a card game, or for better luck with the ladies.
Although otoliths are present in the skulls of most species of fish, they are particularly large in sheepshead, where they are used for hearing and balance. Fish grow otoliths in much the same way as an oyster grows a pearl, building them a layer at a time from a mineral known as aragonite. In this way otoliths are similar to both tree trunks and pearls; as a fish grows, so does its ear stone, providing those who know how to read it with some very interesting information.
Unlike other parts of a fish's body, otoliths are very persistent and have been found in layers of sediment up to 155 million years old. In recent years, scientists have found that reading the rings of an otolith can provide information on the age, health, and growth rates of a number of species of fish, and trace elements in the rings can reveal the temperature and chemical makeup of the water in which the fish lived. Researchers from Bowling Green University, for instance, can tell where a fish lived throughout its lifetime by studying levels of strontium, magnesium and other chemicals within the layers of an otolith, and scientists from the University of Minnesota have even used them to provide information about the climate of the Great Lakes region over thousands of years.