People who can't keep their metaphors straight: "Cast in stone" should be "cast in concrete" or "carved in stone". How would one cast something in stone (I don't think lava is castable).
Intentionally mixing metaphors to confuse people is one of my favourite pastimes.
My favourite one is "Joka toiselle kuoppaa kaivaa, parhaiten nauraa". It's a mash-up of two:
"Joka toiselle kuoppaa kaivaa, siihen itse lankeaa" – He who digs a pit for another will fall into it himself
"Joka viimeksi nauraa, se parhaiten nauraa" – He who laughs last laughs best
Besides, certain types of lava is actually castable. Felsic lava, for example, produces obsidian, which is semi-castable.
It is also important to realize that "casting in stone" does not necessarily require high temperatures; only that the material is initially liquid, then turns into stone. This is how sedimentary rocks like sandstone are formed, after all.