“Y-you mean the rock?” she managed. “This rock is called schist?”
“Yes, greenstone! Schist!” the first creature yelled. “Nasty rock.”
Hazel began to understand how she’d summoned it. “It’s a precious stone. It’s valuable?”
“Bah!” said the one in the yellow loincloth. “Foolish native people made jewelry from it, yes. Valuable? Maybe. Not as good as wheat.”
“Or sorghum!”
“Or barley!”

The Son of Neptune, Chapter XIX