《天工开物·下篇·五金》
礁砂形状像煤炭，底下垫着石头因而显得不那么黑。礁砂的品质分几个等级。
其礁砂形如煤炭，底衬石而不甚黑，其高下有数等。
The reef and sand were shaped like coal and were not very black because there were stones underneath. The reef and sand were graded into several grades.