14k gold on jade tarnished

by Judith
(Brisbane, Australia)

just wondering why the gold tarnished on my jade jewelry. it is a 1930/1940 jewelry; it's a flat jade.

On one side had the chinese long life symbol, on the other side had the good luck symbol. all the side including bale except for the long life symbol tarnished last friday. cleaned it with toothpaste this morning and the gold colour came back.