Vijay Jewellery - I am not satisfied with the gold portion in a jeewelry purchased

Dear Sir,

I have purchased a chain of around 10 gms from the shop (owner Pawan) in 2008 and the owner charged for 24 carets but he did not give any bill as he said that no body asks for bill and if you will take the bill you have to pay 12% extra.
after few months the chain colour got changed and took some blackish colour .we showed the shopkeeper Mr Pawan . but he said that there is no fault but now when we got it checked it is only 68 to 75% gold . now we have been cheated by the shopkeeper.
Now when we asked him to change the chain he asked us to take our money back . but the gold rates has rised to 7-8 thousand how can we accept that amount.
the shop keeper started abusing us and told that do whatever you can.

regds