The Collaborative International Dictionary of English v.0.48:

Grit \Grit\, n. [OE, greet, greot, sand, gravel, AS. gre['o]t
   grit, sant, dust; akin to OS griott, OFries. gret gravel,
   OHG. grioz, G. griess, Icel. grj[=o]t, and to E. groats,
   grout. See Groats, Grout, and cf. Grail gravel.]
   1. Sand or gravel; rough, hard particles.
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   2. The coarse part of meal.
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   3. pl. Grain, esp. oats or wheat, hulled and coarsely ground;
      in high milling, fragments of cracked wheat smaller than
      groats.
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   4. (Geol.) A hard, coarse-grained siliceous sandstone; as,
      millstone grit; -- called also gritrock and gritstone.
      The name is also applied to a finer sharp-grained
      sandstone; as, grindstone grit.
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   5. Structure, as adapted to grind or sharpen; as, a hone of
      good grit.
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   6. Firmness of mind; invincible spirit; unyielding courage;
      fortitude. --C. Reade. --E. P. Whipple.
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The Collaborative International Dictionary of English v.0.48:

Gritrock \Grit"rock`\ (gr[i^]t"r[o^]k`), Gritstone \Grit"stone`\
   (-st[=o]n`)n. (Geol.)
   See Grit, n., 4.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gritstone
    n 1: a hard coarse-grained siliceous sandstone [syn: grit,
         gritrock, gritstone]