We are told in Parashah Mishpatim (Exodus: 24:10) that Moshe beheld the throne of God: "...and under His feet was something like a sapphire brick." This unusual object is later divided into two separate 'stones' and brought down as the shnei luchot (or 'two tablets' of the covenant). Sapphire Magen David : The Sapphire Cube
This article goes on to show more approaches to the appropriateness of the concept that the Ten Commandments were written on sapphire cubes.
This article goes on to show more approaches to the appropriateness of the concept that the Ten Commandments were written on sapphire cubes.