Question

A diamond knife is quite often used for cuting glas. Why?

Answer 1

This is because diamond is the hardest known substance ever found on earth and can cut materials like glass quite easily.
 : ) 

Answer 2

As diamonds are harder than glasses............and Diamonds are considered as the hardest substances of nature.Seventy-five percent of diamonds are employed in industry for cutting tools,drill bits etc.

Hope this helped you and this is answered by my mother so no copying is done from internet or books.............