Pearls are the result of the secretion of calcium carbonate by various
shellfish around sand grains, parasitic organisms, shell fragments, or
other foreign objects that have in some way entered the body cavity of
the shellfish. Since the shellfish is unable to expel these irritating
particles or organisms, it deposits successive layers of calcium
carbonate around the foreign substance to make it smoother and less
irritating. Although pearls are principally calcium carbonate, they also
contain small amounts of an organic substance, called conchiolin, and
water. Pearls are found in shellfish that live in either fresh or salt
water. Few pearls are spherical in shape; most are rounded but somewhat
irregular and are known as baroque pearls. Good quality pearls are the
only gemstone commonly sold by the grain, a unit of weight equal to 0.25
carat or 0.05 gram. The pearl grain is not the same unit of weight as
the Troy grain.
In Texas, pearls have been found in fresh-water clams in most of the
major rivers and streams, notably in the Brazos, Concho, Colorado,
Guadalupe, Llano, Nueces, Sabine, Rio Grande, and Trinity Rivers.
Several Texas lakes have also yielded pearls, notably Caddo Lake and
other lakes in north-central and northeast Texas.
Small pearls are frequently found along the Texas Gulf Coast in edible
oysters and other common shellfish. Fossil pearls have also been found
but because of their darkened appearance are of value only as
curiosities.
The pearls thus far found in Texas have been of relatively poor quality
and show little or no iridescence. These pearls have little value except
as curiosities, although one writer has stated that the discovery of
pearls in the Nueces River led to the original Spanish settlement of the
State (Baker, 1935, p. 569).