Fyrffytr1 is right, an early US Marine Corps button. Being a brass 1-piece Marines button, it dates from the 1820s into the 1830s. The button book by Alphaeus H. Albert, "Record Of American Uniform And Historical Buttons" shows several variations (buttons #MC2, MC3, MC4) of the emblem on your button, but we can't tell for sure which variation you found because there's so much encrustation obscuring parts of the emblem. For example, I can't see whether the emblem is on a lined field or a smooth field, nor can I see any stars. Is the emblem on yours completely surrounded by stars, or only half-surrounded?
Based on being able to see the word "Gilt" in your button's backmark, I think your button is Albert button #MC2 -- which shows the emblem on "an irregularly lined" field.