The Old Testament refers to Abraham (originally Abram) building altars in several locations. The materials used to construct these altars are not explicitly stated in detail, but we can infer some things based on the context and common practices of the time:
* Stones: The most common material for building altars in the Old Testament was natural, uncut stones. Deuteronomy 27:5-6 commands that altars be built of uncut stones, saying "And there shalt thou build an altar unto the Lord thy God, an altar of stones: thou shalt lift up no iron tool upon them." This suggests that the use of unhewn stones was significant for religious reasons, perhaps to maintain the altar's purity or to emphasize its natural connection to the earth.
* Earth: Exodus 20:24 states, "An altar of earth you shall make for Me, and you shall sacrifice on it your burnt offerings and your peace offerings, your sheep and your oxen. In every place where I record My name I will come to you, and I will bless you." So, altars could also be made of earth.
* Likely Simple Structures: Given Abraham's nomadic lifestyle, it's likely his altars were simple structures, probably just piles of stones or mounds of earth.
While the Bible doesn't provide exact details on the specific stones or methods Abraham used, the general understanding is that his altars were constructed from readily available, natural materials, in accordance with the customs and divine instructions of the time.