And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it. Exodus 20:25
Iron is one of the metals which rusts. And it was looked upon as a symbol of pollution. Gold, on the other hand, was the symbol of glory and purity.
The altar was to be built of unhewn whole stones, in Aramaic called shalmatha, "whole" or "perfect." Shalam means "peace." Iron instruments might cause injuries to the workers and pollute the altar with human blood. The stones were to be natural, symbolizing the purity, naturalness, and simplicity of the religion of Israel, as compared with man-made pagan religions.
The altars of the God of Israel were to be different from lavish pagan altars. That is to say, they were to be simple. Many of the ancient altars in the countryside were built of unhewn stones
Lamsa, George M. 1964. Old Testament Light. San Francisco: Harper Collins., pg 142