These are the ancient descendants of the Ko-Boldes who sided with the gods of darkness when the Blood of Saemon crossed over into Helca. They are under the Sun Bane. Enslaved and used nearly to extinction by Orrish and humanoid races of Helca, eventually their Ducateon brethren sheltered the last of them.
They have been forgotten for thousands of years, and only about 20,000 exist in carefully controlled environments by their larger cousins. Almost no one knows what they are now, and their stoney skin protects them best against the Sun Bane for longer than anything except trolls.
Enslaved Miners: The Orrish of today never has good tunneling tools. For thousands of years they have used most of the tools found in the Fallen East of the Steel Realms (Osteni). They are not good miners, they mostly use enslaved Kobolds and Trolls, though the later are huge and hard to control with their bloodlust and short fuse.