The answer is: A stone like a great millstone

Rev 18 vs 21

In Revelation 18:21, we are presented with a striking image—a mighty angel flinging a millstone into the sea to symbolize the absolute destruction of Babylon. Babylon here isn’t just an ancient city; it encapsulates hubris, corruption, and greed throughout history. Throwing a millstone, an object heavy and imposing, into the sea indicates a decisive end—there's no way back for this great metaphorical city.

Now, why a millstone? In ancient agrarian societies, a millstone was essential for grinding grain, but it also weighed a ton. This reflects the burden of sins Babylon laid on the world. Millstones held grave significance in Jesus’ teachings too. He referred to them when discussing the severity of leading others astray—proposing that it would be better to have such a stone tied around someone’s neck and thrown into the sea than to lead a child to ruin (Matthew 18:6).

An interesting trivia tidbit? The fall of Babylon in Revelation mirrors the biblical motif—redos of eras from the golden age of Israel to the material decline of proud nations. Through Marduk, Babylon worshipped gods of power, but here we see a rehearsal of these fallen greats meeting their doom. Woven through the rhythm of prophetic verses, the destruction laid bare serves as timely warnings against the seductive nature of power and excess. Keep this image in mind: the crashing of that millstone reverberating through the narratives of fallen empires.