I was under the assumption that they were augmented humans. After checking the wiki, apparently there's so little information about them that they just say:

According to dimly remembered legends and the few scraps of actual historical record that survived the Age of Strife, the Men of Stone were an artificially-created or augmented faction or sub-species of Humanity who had been engineered by the equally legendary and poorly understood Men of Gold.

Neat. Today I learned. Basically just "Who the fuck knows"