16) What type of natural abrasives are crushed to produce abrasive grains for making the grinding wheels and lapping compound?

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?