What I was leaning towards is the idea that, effectively, the "shape" of a Mountain Folk doesn't matter. They're almost a golem-race, so to speak; their form is set by their statue-body which their heart is placed in at 'birth'. They don't have a childhood - they're made in their adult form. And there's nothing about them which means they have to be humanoid. Sure, Mountain Folk made to interact with surface worlders are built to resemble statues of humans, but that's no more them than the spider-like mining creatures who gnaw at the rock to extract gems. They're both, equally, Mountain Folk - because they're living statues animated by the heart which is a calcified fae.
When they were the People of Adamant, they all used bodies of perfect, flawless adamant. They made no mistakes. Each progeny they crafted was perfect - taking ages to craft - and possessed their full capacity. They made no progeny who were inferior. In the Primordial War, to make up casualties they made the first "slave soldiers" - made from inferior jade, less attuned to their nature, but the jade-born were only a short-term solution to the losses of the war. They were inferior, and they were never meant to make their own progeny. Jade clouded their minds and meant that errors crept in. If the jade-born were to make progeny, they might be even worse.
Only the Exalted came to trust the jade-born more than the distant, cold, calculating adamant-born. They had fought alongside the jade-born, while they could read the actions of the adamant-born after the war and they knew that the People of Adamant would not consider themselves servants of the Exalted - only peers, at best. And they, too, were outraged by how their blood-brother People of Jade were treated by their makers.
So the Exalted Host acted, and the People of Adamant were no more. No wonder Autochthon fled.
But the jade-born realised that they were never meant to be, that they were flawed copies of the People of Adamant. Errors crept in. Within a thousand years, some jade-born were so flawed, so clumsy that they could only work in lesser materials like marble and other stones. The sickening society of the jade-born tried to stop such flawed jade-born from furthering their decline, repeating the sins of their forefathers. They forced quality controls, prevented the construction of new jade-born without vetting, and desperately tried to keep as many of the elder jade-born alive. They begged the Deliberative to try to help them make more adamant-born, to lessen the decline. But the Deliberative was overthrown and the jade-born found that they were presented once again with a choice - swear allegiance to the Shogunate, or face another pogrom.
They swore allegiance.
The Balorian Crusade and the Contagion together irrevocably shattered the Mountain Folk society. The plague wiped out most of their cities, but then the Balorian Crusade trapped vast numbers of new hearts of calcified chaos princes. In the scattered post-apocalyptic remnants of their societies, all the constraints that had maintained their standards were cast aside. Soon the jade-born were a rare elite among the stone-born masses.
Shambling, clumsy, crude - the stone-born are mockeries compared to the grace of the jade-born, and even they were just parodies of the adamant-born. Their minds are limited by their mediums. The elite caste of jade-born try to preserve and expand their ranks, but the stone-born outnumber them and there are ruined cities below the earth where stone-born shamble through, carrying crude club-shovels and following their mountain-deep instincts to make things. Their minds can still contemplate beautiful things, but much of the original savant-like genius is lost to them. Some of the brighter stone-born have learned their old magics which let them alter their bodies and they fortify their selves, but the Mountain Folk will never regain their old glory.
Narratively, the Mountain Folk can play the traditional role of fantasy orcs, dwarves or trolls as the GM sees fit - because they're all of them. Also, they can be giant stone spiders living beneath the earth. They turn to unliving stone in the sunlight, so they're certainly part troll there, but they have a genius with tools and can be cunning even when they're not that bright (somewhat orcish) and the jade-born provide the dwarf-like 'dying race' thing.
