This happens when Ruby becomes disappointed with Tom when the hose leaks and Ruby can no longer dive. The two stop hanging out, and Tom loses her friendship.
The light, then, is the light that her friendship brought to his life. Her friendship is light -- it is happiness and is joy to Tom. When the two friends stop hanging out, the light goes out of the marsh. The author is saying the joy has gone out of Tom's life.
Something that tarnishes easily is metal !
