Maimonides Minute – Hilchot Beit HaBechirah 1:13-16

The Torah commands us to build an altar out of “adamah,” which presumably means dirt or clay. The Torah also says, “If you build it out of stones,” implying that using stones is optional, rather than mandatory. Why, then, do the Sages say that one must use stones for the altar, and that other materials are unacceptable?.