One table was made out of marble (others say silver that looked like marble) and the second table was specifically made of gold, because of the principle that "we increase in the level of holiness and do not decrease." Since the loaves were already on a golden table in the Kodesh, they could not be removed and placed on a table of lesser splendor. (Source: Menahot 11:7). 
The Two Small Openings
Contents:   Part I,    Part II,    Part III,    Part IV,    Part V,    Part VI,    Part VII