We read in the Parsha (Torah portion) the following:
They shall make an ark of acacia wood ( Atzei Shittim), two and a half cubits its length, a cubit and a half its width, and a cubit and a half its height.. And you shall overlay it with pure gold; from inside and from outside you shall overlay it, and you shall make upon it a golden crown all around.( Exodus 25:10-11)
The people were being commanded to place the most precious item in the world into this vessel they were to create. The very words of G-d carved into stone were to be placed into an ark made of acacia wood lined with gold. On the one hand the menorah representing G-d’s light was to be made of pure gold (ibid 25:31) and on the other these precious tablets were to be placed in an ark made of wood that was simply lined with gold.