Silicon nodules were eggs laid by the Horta.
In 2267, millions of these spherical nodules were found beneath the surface of the planet Janus VI. Miners of the Janus VI colony destroyed thousands of these nodules, believing them to be nothing more than silicon rocks with no value. This resulted in the mother Horta going on a murderous rampage, killing over 50 people in the colony. Captain James T. Kirk discovered a hatchery, also referred to as a nursery, inside the Vault of Tomorrow which contained about a million silicon nodules about to hatch. (TOS: "The Devil in the Dark")