Khazana Jewellery — after a year taken necklace turned into brown into copper color

Address:

Khazana jewellers, near etc bus stand, Anantapur, Andhra Pradesh, 515001

I have taken a neck piece from khazana jewellers, i have used it just for couple of times that for an hour each time. After one and half year it turned to color of copper... It's disgusting... I feel like bought a copper ornament. I'm very upset with this... I want to take some action on this.. Looking forward for a justification.
It's been a 4 years of relationship with khazana in buying jewellery. No doubt, i wouldn't be continuing buying here.

Today i will go to the store and complaint about this.. May be it's a small issue for them... But for me it's not.

Regards,
Rehana