I... what is Luke actually holding up?

Spherical candy?

It's the Log Pose, Oda's home-brew compass that points to equator islands instead of planetary poles.

I should have picked up on you saying Luke instead of Luffy. If I ever get into tabletop gaming, I'm going to offer my Lego collection for game pieces and I head-canon that as default for these comics until proven otherwise.

Sketch of a compass pointing at an island instead of north?

Oh my...

Log. Pose.

to put it simply, each Island on the Grand Line has its own, powerful magnetic field.

The log pose, while it sort of looks like a 3D compass, points to these fields instead. One island at a time.

You sail from point a to b. Stay there till the compass 'absrobs' that islands charge. It then points to a new islands.