Glass Rocks
Big rocks of glass in the Land of the Mangaboos, piled up to seal the travellers into the mountain.
First named in Dorothy and the Wizard in Oz, ch. 7
they began to pile big glass rocks within the entrance
What the books say it is
made of glass 1 chapter
they began to pile big glass rocks within the entrance, so that the prisoners could not get out again.
Dorothy and the Wizard in Oz, ch. 7