I was surfing around and up came a banner ad: "Remember how much time you spent in school learning about the isosceles triangle, didactic rock... etc?" I lost the ad in my haste to google the latter term, so I can't tell you any more about it. My search obtained a mere seven hits which led to only two ideas: didactic rock gardens and didactic rock music (!?).

Was the ad writer thinking of something else?

well, nothing comes up at allmusic.com, so I think someone's trying to be creative...

Well, I'm thinking "didactic rock" is some sort of geological term that someone got confused with something else.

it actually could be a pretty good genre...

maybe Schoolhouse Rock fits...

Probably best applicable to bands that churn out albums year after year, each sounding exactly like its predecessor ... ?

wouldn't that be did'at rock?

thinking of something else? It could be a teachers sigh facing a class who seem unable to "dig it".

diamond rock?
detrital rock!?
duck on a rock??