Boyd’s Bible Dictionary:

(little pears). Stony secretions of the pearl-oyster. Reckoned as gems and highly prized as ornaments. Source of frequent metaphor (Matt. 13:45; 1 Tim. 2:9; Rev. 17:4; 21:21). Pearl (Job 28:18), should be crystal.

Concise Bible Dictionary:

Ice Crystals
In Job 28:18 the word is gabish, which signifies “ice” and hence “crystal.” In the New Testament μαργαρἰτης is from “to glisten, shine,” and perhaps refers to pearls, such as are discovered in shells of various species. They are mentioned three times as distinct from precious stones (Rev. 17:4; Rev. 18:12,16). They were worn as an ornament by women (1 Tim. 2:9). Metaphorically the term applies to anything costly; things which should not be cast before swine (Matt. 7:6). The gates of the heavenly Jerusalem were each of one pearl (Rev. 21:21). In the parable of the one Pearl of Great Price the Lord is represented as selling all that He had (as man and Messiah) in order to become its possessor (Matt. 13:45-46). It implies the unique character of the church in the eyes of Christ.
Pearl Ring