And i personally like to think that with the leftover gold dust and the smallest bit of platinum he could manage from treasure island, Senku creates some kind of totem or charm to remind him of Byakuya, his dad. Because even after all that time, the life he created after the petrification, he never stopped thinking about his son, about Senku.
He spent the rest of his life building, planning everything out for Senku to find without ever knowing if he would see him again because he believed in him so much, trusted that he’d be able to rebuild civilization.
So to remember that, the last gift he would ever receive from his father, he creates something. It feels childish, but he keeps it on his person at all times.