I don't believe that Alexander Calder ever created a bookmark...
but if he had, it might have looked like this.

Bookmark

Hand-made from stainless steel wire, heavy gage sheet metal, durable acrylic paint,
and signed by the artist; this bookmark has been designed to capture the essence
of Alexander Calder's unique style with elements of both his hanging mobiles and jewelry.

This bookmark measures 7 inches long