A long time ago, long before I was at the table, Wrathian, one of the more powerful characters, accidentally summoned Ravenloft (an alternate dimension) and it destroyed the city of Greyhawk. Several characters tanked this destruction.
Greyhawk has a population of 160,000. This is closest to Alexandria and Lancaster in the US. So we'll use their land areas.
Low-End: 38800000 m^2
High-End: 244200000 m^2
Average elevation can't be used, since Greyhawk is near the coast, so 50m will do since it is so close to water.
Low-End Volume: 1.940e9 m^3
High-End Volume: 1.221e10 m^3
Density of stone is 2750 kg/m^3, so let's get mass.
Low-End Mass: 5.335e12 kg
High-End Mass: 3.358e13 kg
This stone was dropped from above the city onto it AFAIK. So 150 meter height should work to be somewhat taller than the buildings of the city.
PE = mgh
Low-End: 7.843e15 Joules, 1.88 Megatons of TNT, Small City level
High-End: 4.936e16 Joules, 11.80 Megatons of TNT, City level
I prefer the high-end, but sadly this is much lower than expected.
Wrathian Busts the City (Low-End): 1.88 Megatons of TNT, Small City level
Wrathian Busts the City (High-End): 11.80 Megatons of TNT, City level