'Head stone' in the Bible

What is that headstone I see over there? And the men of the town said to him, It is the resting-place of the man of God who came from Judah and gave word of all these things which you have done to the altar of Beth-el.

Who are you, O great mountain? before Zerubbabel you will become level: and he will let all see the headstone, with cries of Grace, grace, to it.