coal is also a form of carbon and diamond is also a form of carbon then why diamond is too precious?

Coal, an amorphous form of carbon, is not pure form of carbon. It contains organic impurities.  

Diamonds are the well-known crystalline form of carbon. They are very pure form of carbon and formed deep inside the earth, the arrangement of carbon in diamond also matters. In diamonds each carbon atom is linked to four others in a tetrahedral manner and they form a pyramid-like structure. Diamonds does not absorb light of different wavelengths and are transparent and sparkling. 

The purity of diamond and its structural arrangement makes them more precious than coal.