The two photos below are two grave sites in Stafford,Virginia @ Aquia Church, dating to the 18th century. The skulls i think represent a symbol of resurrection on these tombstones. A friend of mine dug a similar button to yours awhile back that i remember that had a skull on the button and a design below the skull and the button dated to about late 18th century. I can't say what your button truly is, not a military button but some sort of civilian type button. I believe from looking at the shank of your button and the construction of the button - that it would date sometime in the 1780s-1820s era.