Where Do Pearls Come From? Rarely a Grain of Sand

Natural pearls form around “an irritant such as a piece of sand or a parasite that has invaded its shell,” according to the Gemological Institute of America’s June birthstone page. The popular explanation keeps only the sand: a grain slips between an oyster’s shells, the oyster coats the irritant in layer after layer of mother-of-pearl, and in time the grit becomes a pearl.

Of the two suspects, sand is the one that rarely turns up. George Frederick Kunz and Charles Hugh Stevenson’s The Book of the Pearl (1908) reported that examinations of many gem pearls “have failed, except in rare instances, to reveal a foreign nucleus of sand or similar inorganic substance.”

Where do pearls come from?

Pearls come from mollusks, chiefly pearl oysters in the sea and mussels in fresh water. A pearl grows inside a pearl sac, a pocket of the cells that build the shell, formed in the animal’s soft tissue, often after a parasite or an injury and only rarely around a grain of sand. Today the vast majority of pearls sold are cultured on farms.

Sand can seed a pearl, but rarely does

Kunz and Stevenson credit the irritant idea to the physicist Reaumur, whose 1717 paper for the French Academy of Sciences, in their summary, held that pearls are “misplaced pieces of organized shell”, set off by “the intrusion of some foreign or irritating substance.”

Sand is not ruled out. The book accepts a hard intruder, “such as a grain of sand, a particle of shell”, as the cause of “many of the baroque or irregular pearls” and of the pearly blisters stuck to the inside of shells. A 1991 training manual from a UN Food and Agriculture Organization seafarming project lists “sand grains” first among possible nuclei, ahead of parasites, eggs and the animal’s own cells.

Every pearl grows inside a pearl sac

The sac is made of epithelium, the cell layer covering the mantle. The mantle is the sheet of tissue that lays down the shell. When a foreign body lodges between the shell and the mantle, the FAO manual explains, the mantle’s outer epithelium folds around it and forms a sac, and the sac’s cells secrete nacre over it in concentric micro-layers: thin sheets of aragonite, a crystal form of calcium carbonate, bound by an organic material called conchiolin.

Left to chance, this seldom happens: the manual calls natural pearl production “accidental” and says it “occurs very rarely.” It is firm on the sac itself: “Pearls are not produced without the formation of the pearl-sac.”

Parasites have been suspects since 1852

Naturalists argued about pearls for centuries, as they did about how eels reproduce, and Kunz and Stevenson name oyster eggs as probably the most popular explanation from the 15th to the 17th century. The parasite theory grew out of a paper published in 1852 by the Italian naturalist Filippo de Filippi, who linked small pearls in freshwater mussels (Anodonta cygnea) from ponds near Turin to a parasitic worm. He saw the remains of the worm, a trematode, at the centers of the smallest pearls.

In 1901 Raphael Dubois tied pearls in mussels on the French coast to the larvae of another parasite. Putting his verdict into English, Kunz and Stevenson granted “some truth” in the statement that “the most beautiful pearl is only the brilliant sarcophagus of a worm.”

In 1903 W.A. Herdman, who had surveyed the pearl oysters of Ceylon for its colonial government with James Hornell, announced that there “the most important cause is a larval Cestode.” A Cestode is a tapeworm. His team thought it passed from the oysters to the trigger-fishes that eat them and on to rays, and Hornell reported that it set off a pearl only when it died in certain parts of the oyster.

The tapeworm case drew doubts early. Shipley and Hornell soon called the trigger-fish link “a doubtful one”, and in 1912 H. Lyster Jameson judged the tapeworm theory “supported by quite insufficient evidence.”

Genetic evidence for a different worm, in mussels rather than pearl oysters, arrived in 2023. Researchers from Spain and France ground up 16 pearls from blue mussels collected at Tromso, Norway, and in two of them found DNA identical to that of the fluke larvae, Gymnophallus bursicola, infecting the mussels. In Parasitology they called it, as far as they knew, the first time parasite DNA had been extracted and amplified from a pearl’s core.

Farmed pearls grow from another animal’s tissue

Pearl farms start the same process deliberately. A technician cuts mantle tissue from a sacrificed mollusk of the same species and inserts a small piece, together with a shell bead, into the gonad of a host oyster, as GIA describes the process. Freshwater farms often skip the bead and insert pieces of tissue alone into a mussel’s mantle, and the nacre forms around the tissue itself.

A 2001 report in GIA’s journal Gems & Gemology found that the vast majority of Chinese freshwater cultured pearls were grown from tissue alone, in the triangle mussel Hyriopsis cumingii. Growers had once put about 40 pieces of tissue into one mussel, and some by then used 28 or fewer. Each piece is meant to grow its own pearl.

The donor’s cells last the whole process. A 2007 study of Pinctada pearl oysters gave the first molecular evidence for it.

In farmed pearl oysters, cells carrying the donor animal’s genome are still present when the pearl is finished, which the researchers in Marine Biotechnology called “a unique biological cooperation.”

	Natural pearl	Saltwater cultured	Freshwater cultured
Grows in	Pearl oysters, mussels, other mollusks	Pearl oysters (mainly Pinctada)	Mussels
What starts it	Chance: a parasite, an injury, rarely grit	A shell bead plus donor mantle tissue	Donor mantle tissue, often no bead
Main sources today	Small quantities from the old fisheries	Japan, China, Australia, Indonesia, the Philippines, French Polynesia	China

Mikimoto’s first cultured pearls were halves

Kokichi Mikimoto usually gets the credit for the cultured pearl, and the Japan Patent Office counts him among its ten great Japanese inventors. By its account he succeeded in 1893 with semicircular pearls grown from a protuberance inside the shell, and patented them in 1896. Round pearls needed a method that Tatsuhei Mise and Tokichi Nishikawa filed patents for in 1907; Mikimoto, nippon.com notes, joined their work with his own and won a round-pearl patent in 1916.

Others had tried with cruder results, the Western Australian Museum notes. By the 13th century, carved pieces of wood and lead inserted into mussels in China were coming out as nacre-covered figures of Buddha, and by 1761 Carl Linnaeus had put limestone beads into mussel shells and got small, poor-quality pearls.

Whole cultured pearls became commercially important in the 1920s, about the time natural pearl production began to decline, according to GIA. The Atlantic sturgeon behind caviar declined drastically within decades of heavy fishing. Pearl beds took far longer: thousands of years of pearl fishing “decimated the natural pearl beds,” GIA says.

Sand gets the credit in the story, but a pearl is the work of stray shell-making cells, and in a farmed pearl most of those cells came from a different animal, cut out and put there by hand. Strictly FYI.