How much damage for throwing rocks?

This question applies to either OSRIC or to AD&D 1st Ed.

The scenario:

A human fighter with strength of 17 has selected “grenade-like weapons” as one of his weapon proficiencies. He carries a small bag with half a dozen mostly spherical baseball sized rocks for throwing at enemies.

He throws a rock at an ogre and scores a hit (not a critical hit or anything, just a normal hit).

How should the damage be calculated?