Which of the following is not a natural abrasive. 

Explanation:

Abrasives:

• A grinding wheel consists of the abrasive that does the cutting, and the bond that holds the abrasive particles together.
• There are two types of abrasives:
  • Natural abrasive
  • Artificial abrasive
• The natural abrasives are emery and corundum.
• These are impure forms of aluminum oxide.
• Artificial abrasives are silicon carbide and aluminum oxide.
• The abrasives are selected depending on the material being ground.
• ‘Brown’ aluminum oxide is used for general-purpose grinding of tough materials.
• ‘White aluminum oxide is used for grinding ferrous and ferrous alloys.
• ‘Green’ silicon carbide is used for very hard materials with low tensile strength such as cemented carbides.