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?

22) What type of abrasives is used for grinding of very hard materials with low tensile strength?

23) Which main part of the surface grinding machine has a driving mechanism?