I had a full inventory, with lots of gems. I did /cnd all, and they turned into leather boots and came out of my inventory. I got all the gems from hunting shipwrecks.

 

A bunch of various mob drops, mostly leather/foods/feathers and random clutter

gem wise I'm guessing, but:

12 Emeralds
20 Diamonds
20 Lapis
80 Iron Ingots
70 Gold Ingots
100 Coal

 

they dropped on the floor and turned into leather boots, which I didn't have

did you have a full inventory? every spot was taken, and then I /cnd'd and instead of the blocks turning into blocks, they turned into leather boots.