Looks like an old mine cut or european cut -- odd setting if it is a precious stone -- normally on something that large I would expect 4-6 prongs. The nice part is both old mine cut and european cut stones are more in demand now time to go find a diamond tester
The ring is set in 14k and if it's a diamond it looks to be about 5k stone. The stone is set with four prongs and has a amber green look to it! I hope this description helps.
Diamonds can come in all colors from red to blue to black -- it might help to get a top down view of the stone. If the stone is a natual green colored diamond that is pretty rare and worth more than a white diamond -- if it is more of a brown then it is a less expensive diamond (if it is a diamond) -- 11.1 MM round cut is 5cts. Any jewler should have a diamond tester on hand -- it usually just takes a min or two for them to do. Got my fingers crossed for ya !!