• The Pearly Gates
• Pearls of Wisdom
• Do not cast pearls before swine

Just a few examples.

Pearl is the official birthstone for June.  Technically, it's not a stone or a crystal.  It's an organic gem formed in oysters, clams and mussels.  They come in both salt and fresh water varieties.  They are formed when an irritant, such as a grain of sand, gets inside the mollusk.  The mollusk secrets nacre to cover the irritating substance and protect it's delicate tissue.  The nacre hardens to a smooth, lustrous coating that starts to irritate the mollusk again causing it to add another coating of nacre.  it will continue adding nacre and, after a number of coatings, you have a pearl.  They come in a variety of colors, mostly white, pink, gray, brown and black.  Most natural pearls come from the Persian Gulf, the Red Sea, and the coast of India.  Cultured pearls come mostly from Japan. 

Folklore and Healing:

Pearl is said to promote good luck, financial success and love.  The ancient Greeks believed they would promote marital bliss.  The Romans considered them the definitive symbol of wealth and prestige, a concept that seems to have been passed on to the British queens.  They were often associated with tears.  Tears of angels, tears of the moon or tears of dragons in different cultures.  They are also associated with wisdom.  When Jesus advised the apostles to not cast pearls before swine he was speaking in allegory, meaning that they should not waste time trying to teach wisdom to those who won't understand it.
Pearl has been used to treat digestive problems and as an aid to fertility and childbirth.  Ancient doctors sometimes dissolved a pearl in alcohol and had the noble person he was treating drink it.  No wonder only the nobility could afford doctors!