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  1. A commercial name for a building or paving stone of bluish-gray color; specif. a dense, tough, fine-grained, dark blue-gray or slate-gray feldspathic sandstone that splits easily into thin, smooth slabs and that is extensively quarried near the Hudson River in New York State for use as flagstone. The color is due to the presence of fine black and dark-green minerals, chiefly hornblende and chlorite. The term is applied locally to other rocks, such as dark-blue shale and blue limestone. CF: flagstone AGI
  2. A miners' term for chalcanthite.
  3. A term applied locally to rocks such as dark-blue shale, blue limestone, and bluish metabasalt (greenstone).
  4. A highly argillaceous sandstone, of even texture and bedding, formed in a lagoon or lake near the mouth of a stream.

Source: Dictionary of Mining, Mineral, and Related Terms

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