Since ancient times, people have believed in the magical properties of a pectoral cross.
Ancestors said that if such jewelry darkened, then you need to think seriously.
If the illuminated pendant darkens, it may indicate the evil eye, damage. In this case, cleaning helps only temporarily.
Unexpected darkening indicates bad deeds of the owner. For example, if gold darkens, it is a sign of a grave sin that was committed by the one who wears this noble metal.
If a person is faced with a serious illness, then chemical substances are released from the body along with the fluid, which can react with the metal.
As for more rational reasons, it is necessary to take into account that some metals can darken at high humidity, and also if the water they fall into contains certain elements that can oxidize it.