Possibly a person’s heart (Definition may be broader. Also see definition for “Burnt Sacrifices/Tithes/Various Offerings”).
Jeremiah 17:1 (KJV) The sin of Judah is written with a pen of iron, and with the point of a diamond: it is graven upon the table of their heart, and upon the horns of your altars;
- Elijah makes an altar of twelve stones to symbolize the people of Israel.
1 Kings 18:31-32 (KJV) And Elijah took twelve stones, according to the number of the tribes of the sons of Jacob, unto whom the word of the LORD came, saying, Israel shall be thy name:  And with the stones he built an altar in the name of the LORD: and he made a trench about the altar, as great as would contain two measures of seed.
- Altar is said to be made of earth (or at least stone not hewn with a tool — not made with hands)
Exodus 20:24-25 (KJV) An altar of earth thou shalt make unto me, and shalt sacrifice thereon thy burnt offerings, and thy peace offerings, thy sheep, and thine oxen: in all places where I record my name I will come unto thee, and I will bless thee.  And if thou wilt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it.