If they are "shiny" and reflective, they are mica, if they are actually brick-like terra-cotta colored, they could be marl - clay-ey inclusions in the stone. Both are not good.
to wit:
<<The old Gibbs rowhouse, at 8 West 128th, has big flakes falling away from the chocolate covered blocks, but most of the brownstone is gloriously unpainted, the whorls and eddys of the sedimentary layers gleaming with specks of mica.>>>
Soon to be playing in a major newspaper near you. yrs truly, Urban Poet
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