Marbles

Planet marbles have been a recent craze and it's completely understandable why. There's just no easy way to beat those natural levels of smoothness and details.

The Earth for example has a height difference of a mere 12.3 km from it's deepest ocean trench to it highest mountain peak. This in combination with a diameter of 12,742 km leaves the Earth really smooth. Together with it's interesting design, it is a highly sought after collectors item.

This is also the reason why the Earth is usually missing when visiting parallel dimensions.