American Express Platinum and Gold cards are stainless steel with polymer back layer. You can see the back layer in bright light.

Start here: https://www.composecure.com/metal-cards scroll down to Award Winning Premium Metal Cards, where you can read a snippet about two Platinum cards and the Rose Gold card. These are all Metal Veneer cards. Read more: https://www.composecure.com/metal-cards/metal-veneer , They also fabricate cards for Chase including Sapphire Reserve which is a Metal Hybrid card, and the discontinued JP Morgan Palladium which was a Full Metal card.