Jetstone is a traditional, militaristic monarchy, made up mostly of dwarves. The Jetstone family has ruled the kingdom for over twenty generations, and it is a rock of stability between two warring superpowers. The current king has expressed his public support of the Harkath monarchy, but focused on border defense during the Harkath/Gastonia war, and has not pledged outright military support if the war should reignite. Jetstone trades extensively with Ecaz, and the inflow of grains and other crops has helped the current Trade King, Daretti, maintain his economic superiority. In addition, the Jetstone dwarves are the only major group probing the depths of the Grand Line mountain range, and they supply most of the ores and precious metals to the continent.
Because many Jetstone cities have been digging downward for centuries, it is one of the few nations that has regular contact with the Underdark. Few outsiders even know of the existence of that dark realm, but it is said that there are constant scuffles far underground with enemies more adapt at underground fighting than the dwarves themselves.
The Jetstone monarchy is the oldest continuous government on Erna, and had existed for centuries prior to the dominance of the Carthic empire. What was once a simple mine run by one family eventually became a haven for dwarves of all stripes. The stone and gems excavated from the mountains fueled the construction of fortified city above and below the ground, with the homes of the ruling family moving deeper and deeper as time went on.
The conquest of the Carthic Empire had very little effect on Jetstone. It's numerically superior armies swept over the land, but met serious resistance when they attempted to breach the fortress-like tunnels of the mines. Cartha initially attempted to siege the mountain strongholds of Jetstone, but pragmatic planning had created stockpiles of food, and the ingenuity of the dwarven engineers led to dozens of secret tunnels. Eventually, a Carthic envoy was sent to the king of Jetstone and a treaty was signed: Jetstone would be largely left alone in exchange for a tithe of minerals and gems, and a Carthic governor would be installed in the aboveground cities of the area.
When the rebellion of Hale Harkonnen swept across the land, Jetstone was wary of joining the fight. Their position favored defense, and even as it became clear that the empire was being pushed back, Jetstone leaders prefered to simply stone wall the armies clashing overhead. However, Hale eventually swayed the king, and two years after the rebellion began, Jetstone forces joined Harkath's. The two nations formed the super majority of the forces facing the empire in open battle, and this shared struggle formed a bond between them which persists to this day.
When the empire eventually withdrew in defeat, both Jetstone and Harkath rushed to expand their borders and influence. Much of the territory of Ecaz was seized, due to that country's passive support of the empire during the rebellion. Eventually, Harkath and Jetstone's expansion ran into the solidified borders of the Fallen Kingdoms, and the modern day borders were established.
Content that their borders were secure, the dwarves of Jetstone attempted to seal their borders and return to the isolation they had known before and during Cartha's reign. However, settlers constantly pushed into the overland territory they had claimed, and in 195AE, Jetstone gave made an offer to all who would settle its land: They would be granted stewardship above the ground in exchange for supplying the below ground cities with food and other goods.
When war broke out between Harkath and Gastonia in 290AE, the king of Jetstone expressed his support for his fellow monarchy, but sent no overt military aid. This left relations with Gastonia frosty but relatively intact as the war cooled off.
There are no outsiders or non-dwarves in the ruling class of Jetstone. The highest levels of power also exclude dwarves not related to the royal family. Once an heir to the king has been named, it is inalienable and those who don't support him are shunned, but the heir doesn't necessarily have to be the son of the king. This leads to a fairly intense period of political maneuvering as the reigning king grows old.
Jetstone has a longstanding alliance with Harkath. As two of the biggest remaining traditional monarchies on Erna, the King of Jetstone and the King of Harkath contribute goods, money, and military to a mutual defense. Jetstone has extensive trade deals with Ecaz for spices, clothes, and other luxury goods.
The kingdom of Jetstone hosts approximately six million souls. Jetstone's largest cities are all underground and all primarily dwarven, making Jetstone one of the least racially and culturally diverse nations on Erna. Jetstone's largest above-ground, Thor Modan, city sits on the southern coast, and though Khaz'Golar is the actual capital of the kingdom, Thor Modan serves as the main settlement for interacting with other races. Non-dwarves are not permitted to set foot in Khaz'Golar, and dwarven noncitizens must go through a lengthy vetting process before they can walk the masterwork architecture of the fabled capital. There are smaller, scattered villages and hamlets on the plains and forests of Jetstone where non-dwarves can find respite, but these are settlements of utility, nothing more.
Jetstone's standing military forces are the smallest on Erna, given the size of its population, but they are also the best trained and best equipped. That, combined with the fact that their underground cities more closely resemble human fortresses than human towns, makes them a difficult foe to attack.
The culture of the Jetstone dwarves is one of isolationism and patience. The affairs of the more flighty races (all of them) are largely ignored, and Jetstone trades only to funnel in luxury goods that cannot be produced in their own underground industry.