21. Once that happens, nothing can be done to restore the nacre , he said. 22. All mollusks can secrete nacre , but oysters do the best job of making pearls. 23. Layer after smooth layer of nacre in concentric circles around the intruder becomes a pearl. 24. In nature, pearl oysters produce pearls by covering a minute invasive object with nacre . 25. These white areas-especially on the pedipalps-have a nacre -like iridescence. 26. The nacre is white and partly iridescent. 27. It is green, brown, or black in color with pink to orange nacre . 28. Researchers there have made an artificial nacre , a composite material that is tough and flexible. 29. This species was used as the subject in a study of the microscopic development of nacre . 30. The nacre is thin and the shells are small, making them of little commercial value.
