A monolithic silicon nitride ceramic is densified at temperatures lower than 2000.degree. C. and heat treated at temperatures greater than 2000.degree. C. in the presence of at least 6.5 w % of multi-component sintering aids. This monolithic silicon nitride has a highly acicular microstructure characterized by .beta.-Si.sub.3 N.sub.4 grains having an average grain width ranging from about 1 to 1.5 .mu.m and average apparent aspect ratio greater than 1.8, Chevron Notch fracture toughness greater than 9 MPa.multidot.m.sup.1/2, R-curve behavior, high Weibull modulus, excellent damage tolerance, high thermal conductivity, and other desirable properties.