It could be a Guinea...
http://www.wildwinds.com/coins/SE/SE3727.html
but this is guesswork without knowing the colour (i.e. metal) and size (diameter) of the "coin".
As an aside, the Latin motto (highly abbreviated on the reverse) reads:
GEORGIVS . III . DEI . GRATIA .
George III by the grace of God
M . B . F . ET . H . REX
King of Great Britain, France and Ireland,
F . D
Defender of the Faith,
B . ET . L . D
Duke of Brunswick and Lueneburg,
S . R . I . A . ET . E
Arch-Treasurer and Elector of the Holy Roman Empire