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.
Seems like the heaviest they produce with contactless is 21g. The old Ritz without contactless was 28g. Was thinking they would bring it back, but looks like its the Metal Hybrid.