from the book, "Petrified Lightning"
In birds, the gizzard is a highly muscular grinding and crushing organ that is part of the alimentary canal. It lies directly behind the organ that secretes gastric juices. Food is mixed with the gastric juices and passed into the gizzard, where it is ground up. The cells lining the gizzard produce a tough horny layer that aids in grinding. The bird will ingest small stones or pebbles (depending on its size) that also aid in grinding the food it swallows.
Some birds perform spectacular gastric feats with their rock—lined gizzards. Ducks and geese scarf up hard nuts, grains, and even live clams; chug them down; and crunch them up with the gizzard's lining of rocks. Clamshells, acorns, and corn kernels are all equally cracked into small pieces by this gastric mill. Pigeons also do very well. Their gizzards are especially tough and contain horn—covered "teeth" that grow from inside the lining. Even the hardest of tropical nuts are swallowed whole, passed into the gizzard, and cracked with an audible thunk.
Large birds, such as the ostrich in the wild, are selective in the stones they swallow. They seem to prefer rocks rich in quartz, and they wander the countryside in search of such potential "teeth." A large ostrich can carry around as much as a double handful of these strong, hard gastric tools.
For the ostriches that inhabited the plains of southwest Africa, choice of stone almost became their undoing. About a century ago a hunter shot a wild ostrich. In preparing it for the evening meal he cut open the gizzard and found several pure gem—quality diamonds among the stony contents. He set out early the next morning to hunt diamond—bearing ostriches. To keep such a find quiet is just about impossible, and word spread quickly. Within a week there was a grand rush onto the plains, and the slaughter began. Prospectors killed the defenseless birds by the thousands. Not all the victims contained diamonds, but some were fantastically rich; in one bird's gizzard 63 diamonds were found.
The ostriches were hunted almost into extinction in this part of Africa. The killing stopped only when too few survivors were left to bother with. The ostrich population has grown since the massacre, but an occasional potential diamond "mine" is still illegally brought down by the ever—present poachers.