Pebble
or PEBBLESTONE, a name given to roundish nodules and geodes, especially of siliceous minerals, such as rock crystal, agate, etc.; but commonly and more correctly applied to small fragments of rocks and minerals which have become rounded and water worn, like the shingle forming the beach on a seashore.
Thus, pebbles may be composed of any rock or mineral. Pebbles of gold are known by the name of nuggets or pepitas.
The term pebble, among opticians, generally means the transparent and colorless rock crystal or quartz (pure silica) which is used as a substitute for glass in spectacles.