Let's see the context, Jonah 1:
3 But Jonah ran away from the Lord and headed for Tarshish. He went down to Joppa, where he found a ship bound for that port. After paying the fare, he went aboard and sailed for Tarshish to flee from the Lord.
They were on a ship.
4 Then the Lord sent a great wind on the sea, and such a violent storm arose that the ship threatened to break up.
Their footing was extremely unstable. They would not be throwing dice onto the ship deck.
7 Then the sailors said to each other, “Come, let us cast lots to find out who is responsible for this calamity.”
Whatever the casting procedure, it was a fair one that was acceptable by everyone.
Proverbs 16:33 sheds some light on this procedure:
The lot is cast into the lap, but its every decision is from the LORD.
The lot is cast into the lap. The bosom or fold of the garment. It is not quite clear what articles the Jews used in their divinations by lot. Probably they employed stones, differing in shape or colour, or having some distinguishing mark. These were placed in a vessel or in the fold of a garment, and drawn or shaken thence. Such a practice has been common in all ages and countries;
Back to Jonah:
They cast lots and the lot fell on Jonah.
Jonah might have just drawn the offending stone from the lap fold garment of the captain.