Sefer HaChinuch ספר החינוך

To not build [with] hewn stones: That we should not build an altar of stones that metal would touch, as it is stated (Exodus 20:22), “do not build of hewn (gazit) stones.” The explanation of hewn (Ramban on Exodus 20:22) is when we chisel from the stone with an iron tool. And if [the altar] is built with hewn stones, it is disqualified.

It is from the roots of this commandment that we should fix into our souls from the day that we make [the altar], that forgiveness of iniquity, blessing and peace will come [through] it afterwards. And hence to remember this thing, we were commanded to not do anything upon it with tools that are fit for destruction — and that is [those made from] iron that cuts and is constantly ready to spill blood. And I have already prefaced for you at the beginning, that man is acted upon according to his actions, and [that] his thoughts always follow his deeds. Therefore, it is fitting for us to do symbolic actions, according to the intention of the things. And when the fool who is [too] rushed hears these things, he will not know nor understand [them].

The laws of the commandment — for example, from where they would bring these stones with which they would build the altar, that they, may their memory be blessed, said (Mishnah Middot 3:4) that they would bring them from virgin ground or from the Great Sea; the law of if metal touched a stone after the altar was built, whether all of it is disqualified or only it alone is disqualified; that which they, may their memory be blessed, said (Mishnah Middot 3:4), [that] when they whitewashed the altar twice a year, that they did not whitewash it with a tool that had iron in it, so that iron would not touch a stone; and the rest of its details — are [all] elucidated in Tractate Middot. (See Mishneh Torah, Laws of The Chosen Temple 1.)

And [it] is practiced at the time of the [Temple] by males and females. And one that transgresses it and builds the altar or [its] ramp with a stone that iron touched is lashed.