Are Pearls costume jewelry or something else?

by Barb Bolstridge
(Leesburg, VA USA)

I purchased a strand of multi-colored pearls at an consignment store.14k gold squares separate the 6 white pearls from the colored ones which are dull gray, shiny gray and cranberry. They have 14k stamped on the insert part of a ball clasp and a letter M with an arrow through the middle of the M. The arrow is pointing up. What is this identifying mark?