The Famous "Conchos River Pearls"... just another native North American Pearly Mussel (found in Texas and also in parts of Mexico): the "Tampico Mussel" as Valeria aptly noted. I have some shells from Tamaulipas (Mexico, south of Texas) and the colors are indeed nice: mostly purple and pinkish.
The "estuary" part could be a plain mistake and they could just refer to a small meander or shallow lake-pond-like area. Never saw them growing near the ocean nor in brackish waters...
The "feeling for pearls" part could really mean "feeling for pearly mussels"...just badly written.
"These Concho pearls are the only naturally colored freshwater pearls in the world.": I guess they mean to say that with most pearls being processed you don't have much certainty about their color...but since theirs are natural pearls they have natural colors. Bad phrasing again.