The Collaborative International Dictionary of English v.0.48:

Slate \Slate\ (sl[=a]t), n. [OE. slat, sclat, OF. esclat a
   shiver, splinter, F. ['e]clat, fr. OF. esclater to shiver, to
   chip, F. ['e]clater, fr. OHG. sleizen to tear, slit, split,
   fr. sl[imac]zan to slit, G. schleissen. See Slit, v. t.,
   and cf. Eclat.]
   1. (Min.) An argillaceous rock which readily splits into thin
      plates; argillite; argillaceous schist.
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   2. Any rock or stone having a slaty structure.
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   3. A prepared piece of such stone. Especially:
      (a) A thin, flat piece, for roofing or covering houses,
          etc.
      (b) A tablet for writing upon.
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   4. An artificial material, resembling slate, and used for the
      above purposes.
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   5. A thin plate of any material; a flake. [Obs.]
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   6. (Politics) A list of candidates, prepared for nomination
      or for election; a list of candidates, or a programme of
      action, devised beforehand. [Cant, U.S.] --Bartlett.
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   Adhesive slate (Min.), a kind of slate of a greenish gray
      color, which absorbs water rapidly, and adheres to the
      tongue; whence the name.

   Aluminous slate, or Alum slate (Min.), a kind of slate
      containing sulphate of alumina, -- used in the manufacture
      of alum.

   Bituminous slate (Min.), a soft species of sectile clay
      slate, impregnated with bitumen.

   Hornblende slate (Min.), a slaty rock, consisting
      essentially of hornblende and feldspar, useful for
      flagging on account of its toughness.

   Slate ax or Slate axe, a mattock with an ax end, used in
      shaping slates for roofs, and making holes in them for the
      nails.

   Slate clay (Geol.), an indurated clay, forming one of the
      alternating beds of the coal measures, consisting of an
      infusible compound of alumina and silica, and often used
      for making fire bricks. --Tomlinson.

   Slate globe, a globe the surface of which is made of an
      artificial slatelike material.

   Slate pencil, a pencil of slate, or of soapstone, used for
      writing on a slate.

   Slate rocks (Min.), rocks which split into thin laminae,
      not necessarily parallel to the stratification; foliated
      rocks.

   Slate spar (Min.), a variety of calcite of silvery white
      luster and of a slaty structure.

   Transparent slate, a plate of translucent material, as
      ground glass, upon which a copy of a picture, placed
      beneath it, can be made by tracing.
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WordNet (r) 3.0 (2006):

slate pencil
    n 1: a pencil of soft slate (or soapstone) used for writing on a
         slate