Jewellery can sometimes discolour the skin due to a non-harmful reaction between the metal in the jewellery and the acids in your skin. Cosmetics, soaps, sweat, and even clothing can alter your ph which can exacerbate it. This reaction can cause the metals in the jewellery to oxidize, resulting in discoloration of your skin. Don't worry, it isn't harmful and will wash off, and is not a reflection of the quality of the jewellery.