Strengthened and toughened ceramic composite bodies are achieved through the use of a silicon boride composition of silicon borides and borides of the form Si-B-C. This strengthening and hardening is achieved when the silicon boride composition is present over a wide range, although a preferred amount is about 20 wt. %. This strengthening and toughening occurs for oxides, carbides, nitrides, and mixtures thereof, for example. One silicon boride composition for this application is silicon hexaboride. Further enhancement is achieved by the addition of up to about 25 wt. % carbon in the silicon hexaboride. Accordingly, the preferred silicon boride composition is a Si-B-C composition, with carbon being added to SiB.sub.6, for example.