There is a story in China about a man and his wife who were forced to be separated. Before they parted, they took a mirror and divided it into two pieces (mirrors weren't usually glass back then and was most likely bronze). They agreed to try and find each other at a certain time in a market.
The man showed up at the market and found his wife's mirror but no wife. Because of strife with war, she had been taken into the house of the conquering army's general, who hoped to woo her. The husband scratched a poem onto the mirror to let his wife know that he was alive and still loved her. When she read the poem she cried tears of both happiness and sadness, joy and despair, for though she rejoiced that he was unharmed, she saw no way out of her current predicament.
However, the general (who really wasn't that bad a guy), saw that she was sad and discovered the whole story. Moved by their love and fidelity, he arranged for the couple to be rejoined.
The phrase, a broken mirror joined anew is used thus to describe a couple who has been parted and then reunited with each other.