Muhammad purified it...
Hi All
From the very earliest days, perhaps even Abraham's time, the Ka'ba has been a shrine. Once triumphant in Mekkah, Muhammad purified it of idols, but kept the stone as a shrine to provide continuity for the Arab people, since Abraham was said to have worshipped there. Being unmade by human hands and from heaven meant it was acceptable as a focus of worship, but only as a sign of Allah's might.
Mind you if it's as heavy as it looks the impact must've been spectacular. Even more of a sign, if anyone had been around.
Adam