Cobblestone is both the name of a planet and its solar system, and is an important trading hub along the Steel Road.
Cobblestone was originally called FUS-944: only solar systems with inhabited worlds are given formal names. The rest are given labels.
The Cobblestone System (often called just "Cobble" for short) was a common stop along trade routes as far back as the Golden Age, when it was a common FTL cool-down location. The system then contained no habitable worlds, and was a popular hunting ground for raiders and space pirates. The lack of any permanent habitation made it difficult to police. During the Golden Age, the Foundation frequently placed patrols in the area.
In the late Golden Age, the Runarins built a large space station orbiting one of the gas giants in the outer solar system, and attempted to station a garrison of fighters there. However, the garrison had little impact on raiding. Most spacecraft using the region for cool-downs avoided the station, which taxed any traveler and charged very high fees for docking. The Runarins' goal of building a trade hub failed.
By the early Second Age, the Runarin government had withdrawn its support. A few private groups attempted to maintain the station, but without access to parts, it quickly fell into disrepair and became uninhabitable, and by A.Y. 2100, was abandoned.
In A.Y. 2904, a Foundation ship (which still made regular patrols) made a fly-by of the abandoned station and noted some intriguing changes. The huge station had been a long, narrow tube with very heavy armor, but had collapsed into a rough, oblate spheroid, and over the eight hundred years it had sat abandoned, had even gathered dust and frozen gasses. While this process was certainly not unexpected (the station relied on 'active' structural integrity to maintain its shape, and had a high mass), it was still interesting to observe none the less, and made the rounds on various "interesting news" sites.
More importantly, it sparked an interesting idea. In A.Y. 8913, a terraformer working for the Foundation's WorldCraft subsidiary made a proposal. FUS-944 had no suitable planets for terraforming, but it had, in the terraformers' own words, "a bunch of other random crap floating around".
Formation of the Cobble System
With its new terraforming fleet under construction in the Markhot System, WorldCraft gathered eighteen ships at FUS-944. Their first act was to move a small, rocky planet from the inner solar system into the habitable zone. Then then moved a second, mostly icy planet from further out, and dragged in various asteroids and other debris. All of these were set on the same orbital path and allowed to collide. The collision of the two small planets was a major event, observed live by over ten thousand starships and around twelve million spectators.
WorldCraft also dragged six moons from gas giants and hundreds of cometary fragments, piling them all together until the mass equaled a standard-sized planet. The mass, aided some by the terraformers, quickly formed a roughly spherical shape. Atmosphere (again taken from the gas giants) was added, orbit and rotation adjusted, and finally, in just twenty years, a planet had been "built".
Terraforming would require another two centuries to complete, but the new planet was quickly named "Cobblestone" for its lumpy shape.
The Founders Society
Around ten thousand of the original colonists could trace their lineage directly to the spectators who had been in the system 220 years earlier to witness the original planetary collision. Together, they formed the Founders Society, a fraternal organization. Over the years they eventually began to accept anyone, but only those with direct blood ties to people who'd seen the formation could hold the title of "seeniter". This is not a malapropism, but a title given to 'those who had seen it', with a further play on the title of 'senator'.
Characteristics of Cobblestone
Planet Cobblestone is unusual for its especially rough surface. A typical planet will not have more than 12 to 20 miles' difference between its highest point and lowest natural points. Cobble sees an average of forty, with some points up to 60.
This leaves the planet with deep oceans and incredibly high mountains. Large regions of the land area have atmosphere that is too thin to breath, while near the coast high pressures cause some problems. Cobble is known for extreme weather conditions.
Also of note is the degree of change. Cobble's surface is dominated by gentle slopes, sometimes uniform over huge distances. There are not many of the very traditional mountain ranges as most people know them.
One advantage of the low-atmosphere regions is that they make ideal space ports. Less atmosphere, less drag, and large areas that aren't good for much else.
Cobble is also home to two orbital elevators.