The incident occurred on Wednesday evening, just before a Tattoo performance was about to get under way.
Historic Scotland, the body responsible for the landmark, said regular fireworks displays and flypasts were not to blame.
It is thought torrential rain could have played a part, along with natural erosion.
The fall is the latest of at least eight in the past decade and the chunk of rock is believed to have come from below the castle's Great Hall.
-- Advertisement --
A spokesman for Historic Scotland told the Scotsman newspaper: "There is no evidence to suggest that firework displays during the Tattoo or other annual events have any effect on the stability of rock faces below Edinburgh Castle.
"We previously undertook a full year of vibration monitoring at the castle to establish if fireworks would have any effects on historic glass. The results showed there were no vibrations from any activity at the castle sufficient to crack glass.
"Given these findings, it is unlikely that vibrations from fireworks have a significant effect on the rate of rockfall. Only four rockfalls have been recorded in the past five years - in September 2008, September 2010, March 2011, and Wednesday's incident.
"The degradation of the rock is an entirely natural process."
A council spokeswoman said: "The maintenance of Edinburgh Castle is a matter for Historic Scotland."
(NE)
