Matthew Henry Commentary Exodus 31:18

"And he gave unto Moses, when he had made an end of communing with him upon mount Sinai, the two tables of the testimony, tables of stone, written with the finger of God." — Exodus 31:18 (ASV)

The law was written in tables of stone. This was to show how lasting it is and likewise to denote the hardness of our hearts; one might more easily write on stone than write anything good on our corrupt natural hearts. It was written with the finger of God, by his will and power. God alone can write his law in the heart. He gives a heart of flesh; then, by his Spirit (who is the finger of God), he writes his will in the heart (2 Corinthians 3:3).