The Collaborative International Dictionary of English v.0.48:

Pebble \Peb"ble\, n. [AS. papolst[=a]n; cf. L. papula pimple,
   mote. See Stone.]
   1. A small roundish piece of stone; especially, a stone worn
      and rounded by the action of water; a pebblestone. "The
      pebbles on the hungry beach." --Shak.
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            As children gathering pebbles on the shore.
                                                  --Milton.
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   2. Transparent and colorless rock crystal; as, Brazilian
      pebble; -- so called by opticians.
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   Pebble powder, slow-burning gunpowder, in large cubical
      grains.

   Scotch pebble, varieties of quartz, as agate, chalcedony,
      etc., obtained from cavities in amygdaloid.
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