Metal Cubes are special items primarily used to increase the time that the explorer can be in another world through a portal. Three metal cubes allow an expedition to spend 100+ days in the Portal World and attain the "Beyond the gate" achievement (spending 100 days in a portal world).
A metal cube can also be used to disable shrine curses by placing it on the altar when looting. Be careful not to use the "Take All" option, as this will take back the cube as well. When using a cube, you still lose standing with the natives.
Metal Cubes can be found randomly for trade at shaman huts, and rarely as loot from caves, shrines and buried treasure.
There is also a rare insanity event that can occur in which a companion disappears and is replaced by a metal cube.