Pearl is also a birthstone for June.
It is one of the few gemstones that has organic origins.
The pearl comes from the shelled mollusk. In nature, when a bivalvial creature, be it an oyster or a clam or some other mollusk gets an irritant in its shell, it begins to secrete a calcium carbonate solution to try to smooth the irritant. When you cut a pearl in half, you will find there are layers of the calcium carbonate and conchiolin. Conchiolin is an organic macromolecule that consists of proteins and polysaccharides that form the microvenviroment where the crystals nucleate and grow. This compound also holds and binds the crystals o aragonite to give their shells their stiffness.
Natural pearls are rare in nature. Hundreds of pearl oysters may have to be harvested and opened to find a single pearl.
Cultured pearls are formed when a selected irritant is placed inside the oyster or bivalve and returned to the water. After a given amount of time, usually 2-3 years, they are retrieved and opened to harvest the newly formed pearl.
By Dlrohrer2003 (Own work) [CC0], via Wikimedia Commons

Chemical formula: CaCO3
Color: white, pink, silver, cream, brown, green, blue, black, yellow, and purple.
Mohs scale: 2.5- 4.5
Streak: White
Pearls are weighed by the Japanese standard of the momme. 1 momme = 3.75 grams or 3750 milligrams. Though millimeter size range is typically the first factor in determining a cultured pearl necklace’s value, the momme weight of pearl necklace will allow the buyer to quickly determine if the necklace is properly proportioned. This is especially true when comparing the larger south sea and Tahitian pearl necklaces.

Famous pearls:
The Abernathy Pearl: A 44-grain natural pearl was found in 1967 by Bill Abernathy in the River Tay in Scotland.
The Abernathy Pearl

The Arco Valley Pearl: Reportedly given to Khubilai Khan, the emperor of China, by Marco Polo, the Arco Valley Pearl is a baroque pearl weighing 575 carats, or 2301 grains, (78 x 41 x 35 mm) and is a white pearl with overtones of pink and brown. It is classified as the 3rd largest nacreous pearl in the world, 1st largest necreous pearl in Asia. It is also considered a blister pearl.
The Arco-Valley Pearl

The Gogibus Pearl: 143 carats and 2 inches by 2 ½ inches and is classified as a pear shaped pearl. It once belonged to the King Philip IV of Spain who wore it as a button on his crown in 1620. It was bought in by Fracois Gogibus in 1620 who sold it to Philip IV of Spain. It has a tight crack extending down as you can see in the photo.
gogibus hand
Gogibus Pearl

The Hope Pearl: Most likely the largest and most famous natural saltwater pearl ever discovered, the Hope Pearl, is an astonishing gem. The pearl weighs 1,800 grains (450 carats), or approximately 4 ounces! The Hope is a white, drop-shaped blister pearl, measuring approximately 2 x 4 inches, and ranging in color from greenish-gold on one end to white on the other.
The Hope Pearl

Macon, Georgia 31210