The rectangular-cut stone is estimated to be 10 karats. A spokesperson said the ring's estimated value is based on the stone's 2.7- by 2.5-centimeter size and the value of other emeralds from Atocha.
Divers from Mel Fisher's Treasures found the ring Thursday about 35 miles from Key West, Fla.
They think it may have come from the 17th century sunken Spanish galleon called the Nuestra Senora de Atocha.
That ship sank off the Florida Keys in a 1622 hurricane.
The divers also found other artifacts including two silver spoons, a 40-inch gold rosary and a gold bar. The last two items were found in March and April, respectively.
The Associated Press contributed to this report.