In a message dated 3/29/2006 3:47:58 PM Eastern Standard Time, [log in to unmask] 
writes:
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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