A variety of pearls produced by several types of fresh-water molluscs, e.g.: mussel pearl produced by the pearl mussel clam pearl, such pearls are usually inferior to the pearl of the pearl oyster; they are of various colours
Fresh-water pearls are found in rivers of Scotland, Canada, the Mississippi River basin, and the Amazon basin. Those from Mississippi region are usually of irregular shapes, hence called 'dogtooth', 'wing', and 'petal'. The largest known fresh-water pearl, owned by W. H. Moore, of Arkansas (USA), weighs 122 ½ grains; the largest from Scotland, found in 1967, weighs 44 ½ grains.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson