The answer is: A stone like a great millstone

Rev 18 vs 21

In Revelation 18:21, the image of a “great millstone” being cast into the sea is a powerful symbol against the backdrop of ancient and modern empires. A millstone isn’t just a massive stone; in biblical times, it was crucial for daily life, used for grinding grain into flour. Its weight suggests a finality—the crushing weight of judgment against Babylon, which represents more than just a city; it exemplifies the corrupting influence of power, greed, and moral decay.

By visioning this act of throwing the millstone into the sea, the writer underscores that Babylon's fall will be total and irreversible, as Christ oversees the demise of ethical corruption symbolized by Babylon. The sea, often reflecting chaos in Hebrew tradition, becomes a witness to the order God will establish.

An interesting layer of Old Testament allusion can be spotted in Jeremiah 51:63-64, where a similar metaphor is used. Such references create a reverberating continuity in scripture about the ultimate downfall of oppressors. It's a hint of divine justice that reassures readers throughout history that nothing built on exploitation lasts.

This imagery has even found its way into modern expressions—the proverbial “millstone around one's neck” reflects the burdensome guilt of wrongdoing, reminding us how centuries later, the ethical principles echoed in these ancient texts still resonate deeply within our collective conscience. So, the great millstone, beyond humor and ancient metaphor, emphasizes the fundamental biblical theme that justice—and maybe some good ol' karma—is on its way.