It's perfectly normal for your Qi-Shield to develop grey/black lines around it - it is simply an effect of the material used. The Qi-Shield is coated with a diffusion-open lacquer so that the surface is not isolated (this is because it is beneficial for your skin to directly touch the copper due to its antimicrobial properties). When your skin touches the Qi-Shield the lines will develop overtime. This has no negative impact on the functioning of the device.