What is it exactly? Dice? Multisided and Colored objects? Evens or Odds? I don't think we really know.
It is interesting that they prayed for God's direction and then cast lots. I think this is so that in the future on one can say, "Well, I didn't vote for him" or "I wanted Joseph". They believed and trusted that God controlled the lots and put in place the one that He had chosen.