What is "pebble powder"

Pebble \Peb"ble\, n. [AS. papolst[=a]n; cf. L. papula pimple, mote. See Stone.]

1. A small roundish stone or bowlder; especially, a stone worn and rounded by the action of water; a pebblestone. ``The pebbles on the hungry beach.''
   --Shak.

   As children gathering pebbles on the shore.
   --Milton.

2. Transparent and colorless rock crystal; as, Brazilian pebble; -- so called by opticians.

   Pebble powder, slow-burning gunpowder, in large cubical grains.

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