Fobos is a perfect illustration for the phrase "space stone", though in reality its diameter exceeds 20 km.
It is worth noting that Fobos is in a literal sense a death satellite. It is slowly approaching Mars. In a few tens of millions of years Fobos will get so close to it that it will be torn apart by gravity. As a result, the Red Planet will be decorated with an extremely spectacular ring. Then the fragments of Fobos will gradually fall on Mars. The space can be cruel.