3,830. The Stones for the Altar

Hilchos Beis HaBechirah 1:13

The altar may only be made of stone. Even though Exodus 20:21 refers to it as “an altar of earth,” the Sages understand that to mean that the altar must be attached to the ground, as opposed to being built on an arch or a cave. And even though verse 20:22 continues “If you make an altar of stone...,” our oral tradition teaches that this is in fact non-optional.

Hilchos Beis HaBechirah 1:14

Let’s say that a stone has been damaged so much that a nail would get stuck in it like a slaughtering knife. Such a stone may not be used in building the altar or its ramp, as per Deuteronomy 27:6: “You shall build the altar of God from whole stones.” The stones for the altar were brought from ground that has not been cultivated. They would dig to a point that was clearly never tilled or built upon and take the stones from there. They could also take stones for building from the Mediterranean Sea. The stones for the Temple and the Azarah were likewise whole.