Pearls are a beautiful organic gemstone which is formed in a variety of colors and shapes.  Pearls form inside of mollusk shells such as oysters and mussels.  This unique gemstone is made of primarily the mineral aragonite.  Aragonite is the mineral that lines the inside of the mollusk shell.  An organic substance, called conchiolin, is also known to line the inside of the shell.  When shell linings are made of aragonite and conchiolin together, it is called mother-of-pearl. 
 
To form the gemstone pearl, a grain of sand must get trapped inside the mollusk shell.  The aragonite forms in circles around the sand grain.  It can take between 2 and 8 years for a large pearl to be formed.  Fairly soft, pearls are a 3 on the Mohs hardness scale and have a white streak.  The luster, "pearly," is often used to describe the look of other minerals with similar outward appearance.  Pearls can form in both freshwater and salt water and can be round and smooth to oblong and uneven.  Common colors of pearls include white, cream and black; however, other colors such as blue, yellow, gray, green, light purple and mauve can also be found.