From today's  "New York Sun"
 
"Developers Mull Uses for Rare Rock Excavated in Tunnels"
 
Refers to Manhattan Schist from Second Avenue Subway tunnels.  So, when is schist - the pigeon guano of New York - rare?   Quotes $100 psf (instead of $15 psf for granite) --- but isn't that because it's not available because of low demand, not actually scarcity?
 
cHRISTOPHER



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