Lily (1986)

Lily Miniver is an eccentric, intrepid young woman whose job as associate curator at the prestigious Jeffersonian Museum in Washington, D.C. takes her around the globe. When a unique Aztec necklace, on loan from the British Museum, is slightly tarnished, she quickly deduces that it's a fake, and she is sent to London by her boss to determine how a substitute necklace showed up at the Jeffersonian.