The Roman History · Amm. 23.6.85

It only remains for me to say a few words about the origin of this stone. Among the Indians and Persians pearls are found in strong white sea-shells, being created at a regular time by the admixture of dew. For the shells, desiring as it were a kind of copulation, open so as to receive moisture from the nocturnal aspersion. Then becoming big they produce little pearls in triplets, or pairs, or unions, which are so called because the shells when scaled often produce only single pearls, which then are larger.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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