means Smooth, lustrous gems formed inside the shells of oysters and other mollusks, prized for jewelry and long treated as symbols of wealth, purity, and refinement.

from From Old French "perle," which passed into Middle English. The deeper root is debated: a common suggestion is Latin "perna," a word for a leg-of-mutton shaped sea mussel, possibly nodding to the shell's shape — but this is uncertain. What's solid is that pearls have carried their name and their value across Europe for many centuries, the word polished smooth by use just like the gem itself.