This is a George III Indian Peace Medal. Made in three sizes, this is the rare (only 9 are known) medium size 2.25 ” (58 mm) diam, medal (Adams 8.1, first of two states). Date: 1776-1800 (most probably issued only during the Revolution). This is a silver shell, joined by a silver rim, the first known example of a shell version of this medium size George III medal (though the large size, Adams 7.1, 11 of 24 are shells) 36.1 grams weight.
Images and description courtesy of the Pavilion Gallery – Americana Art & Antiques at: http://rhbantiques.com/MedalsUSMintother.html