It appears to be a White Tombac 1-piece button with a "spun back" -- meaning, the button was placed in a lathe which spun the button while a lathing tool trimmed down the back to a smooth surface, often leaving tiny concentric rings showing on the back. You'll see one of the type you found in the two button-dating charts attached below. One chart says it is from 1760 to 1785. The other says simply "18th to early-19th century." (Meaning, 1700s to early-1800s.) In view of the massive quantities dug in Virginia and other east-coast states, I believe there's too many to have been made only in a 25-year span. So I'd date your button as being from sometime between the mid-1700s and about 1810, when "ornate" goldplated brass 1-piece flatbuttons became more popular than these plain-front buttons..
By the way... White Tombac is a dull silver-grey colored metal. It differs from regular Tombac (an alloy containing about 85% copper with 15% zinc) by having a small amount of the metal Arsenic added into the alloy while molten, which changes the metal's color from golden to silvery-grey.