Nancy Hudson, age 12, of Shreveport, Laos "for her question:
How do fresh and salt water pearls differ?
The valuable gem pearl is made by a pearl oyster as he sits on the floor of some warm salty sea. But he is not the only pearl maker in the world. Pearls are sometimes made by abalones* by mussels and by many mollusks. The nautilis, who is a relative of the octopus, also makes a pearl and so do many snails. All these are ocean pearls. Some are valuable gems and some are drab and worthless. Fresh water pearls are made by many creatures that live in rivers. Some are lovely enough to be marketed and these are made by various mussels,
All pearls are made from a mineral called aragonite, a form of calcium carbonate. This is the same material that is in the pearl maker's own shell. In the shell, the calcium carbonate is arranged in three layers. The outer layer is a rough, horny protective armor called the conchin. Next is a lovely layer of tiny prisms which break the light into rainbow colors and reflects a glossy surface. Inside is a layer of nacre, or mother of pearl. This smooth, glossy surface lines the inside of the shell,
The pearl is made from the same materials which make the shell, Lustrous pearls are made by creatures with heavy deposits of prisms and nacre, Dull, drab pearls are made by creatures whose shells and pearls are made mostly from co nchin.
The pearl making job begins when an intruder enters inside the shell. It may be a grain of sand or a small worm. The intruder lodges in the mantle, the special layer of living cells which makes the shell. These cells are constantly secreting the mineral aragonite. The substance hardens to form a new coat of paint for the living room walls. The intruder gets caught up in the painting operations.
It is covered with a layer of nacre. Another and another layer is added and a pearl is built.
Fresh and salt water pearls are all formed in this way and from the same basic materials. The salt water pearl makers get their minerals from the sea, where minerals are more plentiful than in fresh water. Perhaps this is why the ocean pearl grows twice as fast as a river pearl. The differences in the finished pearl depend upon how the layers are arranged.
The layers fold around the growing pearl like onion skins. Few of them are complete and most of them overlap. The prism, conchin and nacre layers are often intermixed. Where the layers are mostly conchin, the pearl is dull and drab. Gem pearls and valuable river pearls are coated with countless layers of prisms and nacre.
The makers of fresh water pearls are various mussels that live in many rivers throughout the world. They live in our Mississippi and many mid western rivers. They live in China, Russia and many parts of Europe. In Scotland and Ireland, there are fresh water mussels which make tiny white beads called seed pearls.