What does luck do exactly?

[pink]

*beats Sanko over the head with a stick repeatedly*

Al Rappies drop no such thing as their rare drop. They may drop it randomly if you're lucky, but an Al Rappy is more likely to drop a Mind Material for most IDs on normal The only other rare drops from it are Trimates, Star Atomizers, and Bluefulls get HP Material. So the chances of it dropping a Luck Material are actually slimmer than for most enemies if my calculations are correct.

Just for confirmation... here's the Normal Mode drop chart...
http://faqs.ign.com/articles/453/453744p1.html

Al Rappies only drop rare weapons/armors, etc once you go beyond Normal.

The best place for Luck Materials are Ruins and Seabed. You have a better chance of getting them in those two areas due to the better drops from boxes.