  Frequency standards based on atomic states, such as Rb or Cs vapors, or
single trapped ions, are the most precise measures of time. Here we introduce a
complementary device based on spins in a solid-state system - the
nitrogen-vacancy defect in single crystal diamond. We show that this system has
comparable stability to portable atomic standards and is readily incorporable
as a chip-scale device. Using a pulsed spin-echo technique, we anticipate an
Allan deviation of {\sigma}_y =1E-12 ({\tau})^(-1/2) with current
photoluminescence detection methods and posit exceeding 1E-14 with improved
diamond material processing and nanophotonic engineering.
