I can be worth a lot on one side of a border and less on the other, even though I look the same. What am I?

The book you show to travel, but which is carefully scrutinized before the world allows you in. Solve the riddle → It divides the world's map into invisible segments, and throws your internal clock out of sync when you cross it. Solve the riddle → In an endless circle, it presents unknown twins, while you, with eager eyes, search for your own reflection. Solve the riddle → What was once criticized as an ugly construction, but is now an iron monument that breathes with the seasons? Solve the riddle → An ancient oval in the eternal city, where spectacles of blood and glory once captivated Romans, now stands open to the sky. Solve the riddle →