This is a delicious drink, but I must confess I am confused by the name! As far as I can tell, “Stonage” is not a word.

Is it meant to be “Stone Age”, given that it’s garnished with “edible chocolate stones”?

I’m here so answer my own question.
My research indicates that the recipe was created by Filippo Previero, who is Italian.
Turns out that “Stonage” is Italian for “Stone Age”!
Mystery solved!