An improved fiber reinforced glass composite includes a boron-containing refractory fiber having its surface enriched with boron nitride in a matrix of a black glass ceramic having the empirical formula SiCxOy where x ranges from about 0.5 to about 2.0, preferably 0.9 to 1.6 and y ranges from about 0.5 to 3.0, preferably 0.7 to 1.8. Preferably the black glass ceramic is derived from cyclosiloxane or non-cyclic siloxane monomers containing a vinyl group attached to silicon and/or a hydride-silicon group. The boron nitride-containing fiber is the product of treating the boron-containing fiber with ammonia at a temperature between about 1100.degree. C. and 1250.degree. C. Fibrous failure rather than brittle failure under stress can be obtained.