John Gill Commentary Deuteronomy 4:13

"And he declared unto you his covenant, which he commanded you to perform, even the ten commandments; and he wrote them upon two tables of stone." — Deuteronomy 4:13 (ASV)

And he declared to you his covenant
So the law was called, because it contained, on the part of God, things which he would have done or avoided, to which were annexed promises of long life and happiness in the land he gave them; and they, on their part, agreed to hearken to it, and obey it, (Exodus 24:3Exodus 24:7) ,

which he commanded you to perform, even ten commandments ;
which are seen at large in (Exodus 20:1–17) , and in this book repeated afterwards, (Deuteronomy 5:6–21) ,

and he wrote them upon two tables of stone ;
to denote their durableness; the Targum of Jonathan says they were on tables of sapphire; but it is most likely they were written on tables of marble, since great quantities of it were in Mount Sinai; (See Gill on Exodus 31:18).