Stone
'stone' is the definition.
(I know that pebble is a type of stone)
This is all the clue.
(Other definitions for pebble that I've seen before include "'Small, rounded stone (6)'" , "Stone on beach" , "There are many on a stony beach" , "Little rock" , "one of many on beach" .)
(Other definitions for rock that I've seen before include "Swing" , "Seaside confectionery" , "Sweet < music" , "wobble" , "Lull (baby); 50s pop" .)
'stone' is the definition.
(gemstone)
This is the entire clue.
(Other definitions for opal that I've seen before include "Semi-precious stone of milky or bluish colour" , "Many-coloured gem" , "Gem with delicate colours" , "Precious stone" , "Lustrous stone" .)
'stone' is the definition.
(I know that stone can be written as pit)
This is all the clue.
(Other definitions for pit that I've seen before include "Where the orchestra sits to accompany performers" , "Tip into hole" , "Portland stone" , "A hole as for coal" , "Mine; fruit-stone" .)