https://www.siliconvalley.com/2019/08/22/apple-warns-not-to-keep-its-credit-card-by-leather-or-denim-or-loose-change/

Apple’s new titanium credit card comes with a potential headache that will be familiar to iPhone users everywhere: It can get scuffed up a little too easily in the real world.

Apple updated its support webpage this week to warn customers that some fabrics, such as leather and denim, could cause permanent discoloration to the Apple Card. That damage won’t wash off.