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...