Lifting a LARGE Stone Slab

Our neighbor discovered this stone slab under the dirt floor of her cellar. She dug around it, and then summoned friends and family to lift it out--through a small ground-level window. Here they are moving in to the spot designated for it in her outdoor patio.

The slab measures (roughly) 3 x 22 x 78 inches (8 x 58 x 202 cm). Its weight was estimated by the movers between 400 and 700 pounds (180 to 315 kg)--who would like to try a more precise estimate?

[Edit:] Timon (see below) calculated the slab's weight as 235 kg (= 518 pounds).