Rare Non-Nacreous Purple Pearl, Mercenaria mercenaria,  Northern Quahog clam species,  Coastal waters of the North Atlantic. Estimate: US$ 1,200 - 1,500 (€1,100 - 1,300). Photo: Bonhams.

Described by Linnaeus in 1758, the quahog pearl does not come from an oyster but rather a thick-shelled edible member of the clam family. It is estimated that only 1 in 100,000 Quahog clams actually produce a pearl of any kind. Before its use in jewelry the Quahog was used as the nation's first domestic currency. The Wampanoag tribe used a medium of exchange called wampum which consisted of beads made of the shell of the Quahog bi-valves. 

Ranging in hue from a perfect lavender at its apex to a rich aubergine at its base, this rare non-nacreous pearl if of suitable size and shape for mounting as a unique pendant. 
Weighing approximately 4.29 carats and measuring 9.0 x 7.0 mm

Bonhams. LAPIDARY WORKS OF ART, GEMSTONES AND MINERALS INCLUDING GOLD, 19 May 2015 10:00 PDT - LOS ANGELES