Looks to be a little larger than a quarter,, right ?? Have you tried to polish it up ?? If rusty it has to be iron !! Usually a rosette would be something that could be shined up (silver) as the purpose would be to look good so I doubt that is what it is. My other suggestion would be a ladies purse closure that may have been painted !! Is there a color anywhere on it ?? Whatever it is,, its a great find !! You will have a blast with your detector !! Old church yards are known for having coins found !! Keep looking !! If your town has a historical society,, they might have info on the church and possibly the layout of the grounds !! Around the entrance is a good place to hunt !!