Archaic word for a small stone or pebble; the modern equivalent is 小石 (こいし). The reading せきれき is rare and literary.
See also: 石つぶて (いしつぶて)
In old texts, there is a phrase 'throw a pebble'.
Mixed kana-kanji spelling, likely used when the second kanji is obscure.
The exact derivation is uncertain; the kanji 石 (stone) and 礫 (pebble) combine to mean small stone. The reading せきれき is the on'yomi of both characters.