Here is what Moleskine.com customer service replied.
"Dear Seth,
Thank you for the interest demonstrated in our brand.
We kindly inform you that most of the covers of our collections are made from synthetic materials. More specifically, the hard covers are made in Polypropylene, while the soft covers are made in Polyurethane. Other collections are manufactured in cardboard and some special editions may even include materials such as fabric. We kindly remind you that our paper objects are produced in acid-free paper. For this reason their production is eco-friendly.
Should you have any further questions, do not hesitate to contact us.
Best regards,
Anastasiya
Moleskine Customer Care"