From the report:
But how much titanium? To find out, a Bloomberg Businessweek reporter sent his card to a mineralogist, University of California, Berkeley professor Hans-Rudolf Wenk. Professor Wenk used what’s known as a scanning electron microscope, or SEM device, to determine the card’s atomic makeup. He found that the answer is about 90%. The rest of the card is aluminum, according to the analysis.
Apple previously said the card’s white finish is achieved through a “multi-layer coating process” that is added to the titanium base material. In a recent support document, Apple said if the card comes into contact with hard surfaces or materials, it is possible that the coating can be damaged.
Apple also said some fabrics, like leather and denim, might cause permanent discoloration on the card that will not wash off.