Ross-Simons - 6-12mm Shell Pearl Graduated Three-Strand Necklace, Silver. 18". Get a large, luxe pearl look at an easy price. We've triple-layered luminous strands of 6-12mm shell pearls in graduated lengths of 18", 19" and 21" for a fresh take on the traditional must-have necklace. Our shell pearls are lab-created from oyster shells to ensure durability and affordability. Fastens with a sterling silver box clasp. Layered white shell pearl necklace. Pearl birthstones are the perfect gift for June birthdays.