Discoloration of Gold

Hi i am having a "Gold Haram" which was bought 23 yrs ago by my mom. Now its with me for 151/2 yrs as a gift for my wedding. I am in USA i left my jewels to my MIL and its with her for past 10 yrs and i haven't seen them for last 7 yrs. suddenly she called me and said that Haram became black in few parts and she says that gold is not good and mixed with more" Sembu". But friends says if its true gold became discolored in first few years and will not become like that after 23 years. Is it true? I am very confused and upset. Pls advice me.