“There are no idols in mosques.”
There are any in Catholic churches either.
And ISIS was pretty darn sure the Kaaba stone was an idol -
https://www.facebook.com/261240580751406/photos/isis-we-will-ruin-the-kaaba-after-capturing-saudi-arabiaisisisilwhatever-are-thr/262692303939567/?paipv=0&eav=AfZzODPn22LqlbUBtneNYbEPD2ZlDuitVbcEJsSqm3cHkbPmg3lZMyuajj5qpeG56pM&_rdr
So if Muslims say there is an idol in a Muslim Mosque, you kind of have to go with that don’t you?
I’m just saying. . .
Based on images, my personal opinion is that the Black stone of the kaaba is the black stone ie meteorite that was worshipped as the god Elagabalus in Emesa (present day Homs).