Tafsir of Al-Waqi'ah 56:23

Like unto preserved pearls, indicating that they are just as white and pure fresh pearls. We mentioned Allah's statement,

كٲنهن بيض مكنون

As if they were eggs preserved. (37:49), in Surah As-Saffat, and also their description in Surah Ar-Rahman. This is why Allah said afterwards,

جزاء بما كانوا يعملون