High toughness silicon nitride cutting tools

A silicon nitride cutting tool for cast iron comprising a granular phase consisting essentially of silicon nitride and an intergranular amorphous phase consisting essentially of magnesium oxide, yttrium oxide, and silicon oxide wherein said magnesium oxide is present in an amount from about 0.5 to about 10 weight percent of the total, said yttrium oxide is present in an amount of about 2.5 to about 10 weight percent of the total wherein the ratio of yttrium oxide to magnesium oxide is from about 1:1 to about 7:1, and the balance consists of less than five percent by weight impurities and milling medium.