The answer is: A stone like a great millstone

Rev 18 vs 21

In Revelation 18:21, we witness a dramatic and powerful symbol: an angel throws down a great millstone into the sea, capturing the finality of judgment against Babylon. This throw isn’t merely an act of destruction but represents a profound truth about the consequences of systemic evil and oppression. The image of a millstone – a heavy, grinding stone – suggests that what brings spiritual nourishment can also represent industrial ambition gone awry.

Interestingly, millstones in biblical times varied in weight and marked significant moments in agriculture and everyday life. But in Revelation, their usage spins towards irony. While they typically provide sustenance, in this instance, the millstone embodies the collapse of a great, corrupt civilization. The seas swallowing down that stone underline how the fate of vanity and greed can’t escape divine reckoning.

Moreover, the call of Babylon echoing throughout Scripture, likens it to an ancient epitome of idolatry and moral decay, which was historically referred to in numerous biblical passages. Babel, the city in Genesis where humanity attempted to reach the heavens only to face God's judgment (confusing their language), induces a chilling similarity to this later prophetic commercial hub.

This vivid imagery encapsulates the cyclical nature of human folly, serving as a reminder that all things rooted in exploitation will ultimately face consequence – much like how God’s timeless deeper justice weaves through history as a persistent echo in our ongoing human narrative.