Exodus 20:25

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. If thou wilt make me an altar of stone ... not ... of hewn stone. The reason was, that in the polish of a human chisel the emblematic idea suggested by the earth (see the note at Exodus 20:24) would be entirely lost, while inventions in decorative art would pre-occupy the minds of the worshippers; and an additional reason probably was, lest in following the prevalent styles of architecture, they should be led to carve the altar with figures and ornaments which might lead to superstition; and if once familiarized with such artificial fabrics, they might, on their settlement, appropriate to the worship of God altars that had been besmeared with victims offered to Canaanite idols.