Why were the Ten Commandments given on two tablets? The rest of the Torah was dictated to Moses and written on parchment scrolls. The Decalogue, however, was engraved in stone.
When something is written, the letters and the parchment remain two distinct physical entities. But letters engraved on tablets are part of the stone itself, not separate. This reminds us that we are linked to God on a fundamental level, even though we can sometimes lose sight of that eternal truth. We are engraved within God and Torah is engraved within our souls. Letters carved in stone might be covered in dirt but the letters are still there and cannot be erased.
The tablets, made of blue sapphire (not grey stone), remind us of the Jewish mission: to rectify the physical plane of existence, and honor God by making this world holy.
Image: Moses with the Tablets of the Law by Rembrandt, 1659