EXOD504 Do not use hewn stones to build the Altar. So that we internalize that the Altar brings us peace and blessing, because through the Altar we gain atonement for our sins. To fix this point in our minds and hearts, the Torah forbids us to build an altar out of stones that were cut or shaped by any tools made of materials that are used for destructive purposes, such as iron. Weapons of war and destruction are made of these materials. A tool made of a substance that is used for bloodshed and violence must not be wielded over the stones of the Altar.