  Clock transitions (CTs) in spin systems, which occur at avoided level
crossings, enhance quantum coherence lifetimes T$_2$ because the transition
becomes immune to the decohering effects of magnetic field fluctuations to
first order. We present the first electron-spin resonance (ESR)
characterization of CTs in certain defect-rich silica glasses, noting coherence
times up to 16 $\mu$s at the CTs. We find CT behavior at zero magnetic field in
borosilicate and aluminosilicate glasses, but not in a variety of silica
glasses lacking boron or aluminum. Annealing reduces or eliminates the
zero-field signal. Since boron and aluminum have the same valence and are
acceptors when substituted for silicon, we suggest the observed CT behavior
could be generated by a spin-1 boron vacancy center within the borosilicate
glass, and similarly, an aluminum-vacancy center in the aluminosilicate glass.
