Gold-plated necklaces have a thin layer of gold over a base metal, typically measured in microns. Most last between six months and two years before the plating visibly fades. Gold-filled has a thicker bonded layer and lasts longer, but it is still not solid gold and cannot be resized, repaired, or melted down like a real gold piece. Solid 14k gold is in a different category entirely. It holds its value, can be repaired by any jeweler in the world, and will outlast every other option by decades. It costs more upfront because it is worth more, permanently.