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				Argitoth said:Ghosts have no substance to turn to stone. 
			
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 They have substance, it's just ethereal.  "Ethereal" is not "nonexistent."  Remember, you 
can chop up a ghost with swords, it's just harder to do so.  Petrification is a magical event, not a physical event.
There is a different tag out there that denotes "really really has no substance"; it is the "Truly Etheral" trait possessed by some Doom Horrors.  It provides 75% protection against non-astral magic... presumably this would block Medusa petrification in the same way as it does the spell.