Archaic word for a small stone or pebble. In modern Japanese, 小石 (こいし) is the standard term.
In classical texts, 石子 is read as いしこ and can refer to a small stone.
Compound of 石 (いし, 'stone') and 子 (こ, diminutive suffix). The exact historical derivation is uncertain, but the form is now archaic.