Gold-filled jewelry is not to be confused with gold-plated jewelry. Gold filled has a much thicker layer of gold than gold plating, which prevents the item from flaking off over time.
Gold plated is a thin layer of gold over some other metal, usually brass. It can be distinguished by its brass or silver tint when scratched. The thickness of the plating varies from brand to brand and from piece to piece, but most gold-plated jewelry has a thickness between 0.001 inch (0.025 mm) and 0.0005 inch (0.013 mm).