 Why did the main character no longer keep the ruby necklace?
Images began to appear, drawn from inexhaustible files, wherein are kept all thoughts, past, present, and future.... World War III had been declared, and he had enlisted, receiving his old rank of captain. "It's for you; so long as you wear it, I'll come back, even from the dead, if need be. She took off the locket she wore about her neck and held it up to the shimmering light, letting it spin at the end of its chain. Three days later they had landed in Spain, merged with another division, then crossed the Pyrenees into France, and finally to Paris where the fighting had begun.
 The love that was symbolized by the necklace is now gone.
 He would rather the monster have the necklace.  He wanted the necklace to remain with the house where his love was.  He figured his wife might come back for the necklace and know that he had returned from war.  The love that was symbolized by the necklace is now gone.