Matthew Henry on Exodus 31:18

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