Zakaat on a gold plated necklace

Q: I would like to query regarding 18k gold plated necklace. 

I previously was not aware that zakaat was due on my 18k gold plated necklace. I'm not sure how long I had it in my possession. I am just going to estimate 13 years. I recently also gave the necklace away within this current year. 

I am not sure how much the necklace weight is: will estimate 18 grams, since I am not sure of the percentage/weight of the "gold" in it. 

Will it be okay if I calculate the zakaat on the current selling price of the necklace e.g. The current selling price is R1200 and then to make up for the 13 years =R 15 500 and then pay 2.5% of this amount, will this be allowed ?

Fabricated Hadith regarding the virtue of an Aalim

Q: Someone has mentioned to me that if an aalim passes by a qabrastaan, then the people that are going through difficulties in their graves will get a 40 day break from punishment. The person who told me this, said that this is a hadith. Is this true and is it really a hadith? Is there anything similar to this found anywhere? And what are the true virtues of an aalim?