Altars: To be Made of Earth, or Unhewn Stone
Source: TTT
Exodus 20:24, 25
You are to make for Me an altar of earth, and sacrifice on it your burnt offerings and peace offerings, your sheep and goats and cattle. In every place where I cause My name to be remembered, I will come to you and bless you. / Now if you make an altar of stones for Me, you must not build it with stones shaped by tools; for if you use a chisel on it, you will defile it.
Deuteronomy 27:5, 6
Moreover, you are to build there an altar to the LORD your God, an altar of stones. You must not use any iron tool on them. / You shall build the altar of the LORD your God with uncut stones and offer upon it burnt offerings to the LORD your God.