17) What type of abrasives are used for grinding low tensile strength materials like brass, copper, grey cast iron and aluminium?

18) What type of abrasives are used for grinding high tensile strength materials like carbon steels. high speed steels?

19) What type of natural abrasive contains aluminium oxide and other impurities?

20) What type of natural abrasives contains 95% of natural aluminium oxide and remaining impurities?

21) What type of abrasives is used for grinding of tough materials?