Breeding pearls
N. B. Dennys, a physician and naturalist resident in Singapore, presented this paper to the Singapore branch of the Royal Asiatic Society in February 1878, investigating the widespread local claim that certain pearls can reproduce themselves when sealed in a box with rice. His overarching thesis is one of cautious agnosticism: the positive testimony of multiple independent witnesses is compelling, yet no accepted scientific mechanism accounts for the phenomenon, and he tentatively attributes the observed growth to an insect agency rather than any genuine reproductive property of nacre.
Summary
Dennys reports that “breeding pearls” are reputed to originate chiefly from Borneo and Java, though found throughout the Malayan Archipelago, and that they are extracted from various bivalve species including the giant clam Tridacna gigas (p. 33). The alleged reproductive process is deceptively simple: four or five pearls are sealed in a small box with 15 to 30 grains of uncooked white rice (locally called “Pulot” rice), and after approximately a year, small seed pearls are found strewn across the bottom while the originals have increased in size (p. 32). A distinctive and, to Dennys, diagnostically important feature is that each grain of rice bears a small circular “bite” at its end, the number of bites corresponding exactly to the number of new pearls produced — a mark he is “certain” was produced by a living agency (p. 32).
He marshals testimony from at least six independent witnesses, including a lady whose box had been sealed for twenty years and contained approximately 120 small pearls alongside five originals, and Mrs. ——, wife of Dr. Rowell, the Principal Medical Officer of Singapore, who documented the emergence of twelve pearls from three originals over a twelve-month period in 1874–75 (pp. 32–33). Mr. H. B. Woodford supplied additional detail, noting that breeding pearls change from spherical to irregularly oval when the process begins, that visible layers of scales appear on their surface, and that the full cycle from seed to a quarter-inch pearl may take eight years (p. 34).
Against this body of positive evidence, Dennys sets the negative results of controlled experiments — most notably Dr. Robertson’s sealed-box trial at the Singapore Dispensary, which produced “nil” (p. 35) — and the fundamental scientific objection that hardened nacre cannot biologically reproduce itself. He resolves the tension by proposing that an insect, originally present in the oyster as the foreign irritant that triggered pearl formation, may persist and breed in rice, producing new pearly deposits independently of the original pearls (p. 36).
Key Findings
- Professor Huxley examined one specimen and reported it “absolutely indistinguishable from the ordinary pearl used for jewelry” in all physical and structural tests (p. 32).
- A sealed box containing five pearls and rice, kept shut for approximately twenty years, yielded about 120 new pearls, with every grain of rice bearing a corresponding circular “bite” (p. 32).
- Dr. Robertson’s controlled experiment — four or five pearls sealed with rice at the Singapore Dispensary in the presence of witnesses — produced no new pearls whatsoever (p. 35).
- Mr. Woodford reported that breeding pearls change shape from spherical to irregularly oval during the reproductive phase, with visible laminated scales, and that the full growth cycle to a quarter-inch diameter takes approximately eight years (p. 34).
- The pearls are found embedded close to the valves of bivalve shells, and the largest specimens come from the largest shells, including Tridacna gigas (p. 34).
- After a variable period, breeding pearls “die,” losing their lustrous colour and turning a “dirty flake white” with outer scales peeled off (p. 34).
Conclusion
Dennys ultimately declines to declare the phenomenon either confirmed or refuted. He acknowledges that twenty negative experiments would be outweighed by a single positive one, yet he cannot accept that nacre itself possesses reproductive capacity. His preferred hypothesis — that an insect originally responsible for the pearl’s formation within the oyster continues to act as a pearly-secreting agent in the rice — remains speculative, and he closes by comparing the case to Frank Buckland’s account of a 300-year-old Chinese plate producing mineral crystal growths, conceding that the matter has not advanced beyond “they grew” (p. 37).
Context
- The paper was read before the Singapore branch of the Royal Asiatic Society, reflecting the colony’s role as a hub for natural-historical inquiry in the Straits Settlements during the 1870s.
- Dennys notes that both Sir James Brooke and Admiral Keppel are said to have mentioned breeding pearls in their published works, though he was unable to locate the references in the Raffles Library (p. 37).