In the 1830s, an emerging middle class allowed a greater range of architecture in NYC. While before it was all brick, architects began using brownstone from Hummelstown, Pennsylvania and Portland, Connecticut to build.
Unfortunately, there's no more brownstone left in Hummelstown or Portland. The Times reports that there's simply none left to be mined, and that the last standing facility in Portland will shut down.
While New Yorkers can easily source their brownstone from elsewhere, apparently "stone fabricators and materials experts say that there is really nothing quite like the stone that comes from Portland."