The Greek name of _pearl_ is [Greek: margaritês], originally applied to a precious stone, and apparently moulded out of some oriental name, into a form suited to the Greek pronunciation. Scott and Liddell in v. derive it from the Persian _murwari_. Pliny, _H. N._ ix. 56., speaking of the pearl, says: "Apud Græcos non est, ne apud barbaros quidem inventores ejus, aliud quam margaritæ." The Greek name _Margarita_ was used by the Romans, but the proper Latin name for the pearl was _unio_. Pliny (ibid.) explains this word by saying that each pearl is _unique_, and unlike every other pearl. Ammianus Marcellinus (lib. xxiii. ad fin.) thinks that pearls were called _uniones_, because the best were found single in the shell; Solinus (c. 53.) because they were always found single. The more homely explanation of Salmasius seems, however, to be the true one; namely, that the common word for an onion, growing in a single bulb, was transferred to the pearl (_Exercit. Plin._, pp. 822-4.; Columella _de R. R._ xii. 10.). The ancient meaning of _unio_ is still preserved in the French _ognon_.
Your correspondent asks the "etymon of our English word _pearl_." It would not be uninteresting to learn, at the same time, at what period _pearl_ came into general use as an English word? Burton, who wrote his _Anatomy_ in the reign of James I., uses the word _union_ (from the Latin _unio_) instead of _pearl_ (_Anat. Melanc._, vol. ii. part 2. sec. 3. mem. 3., and ib., p. 2. sec. 4. mem. 1. subs. 4.). In the latter passage he says "Those smaller unions which are found in shells, amongst the Persians and Indians, are very cordial, and most part avail to the exhilaration of the heart."
The Latin term _unio_ differs from "margarita," in so far as it seems to have been applied by Pliny to distinguish the small and ill-shaped pearls, from the large round and perfect, which he calls "margaritæ." And in his ninth book, c. 59., he defines the difference philologically, as well as philosophically. Philemon Holland, who published his translation of Pliny in 1634, about thirteen years after Burton published the first edition of his _Anatomy_, uses the word _pearl_ indifferently as the equivalent both of _margarita_ and _unio_.