Cloudstone is a refined version High Yurisite, one of the two naturally occurring forms of that mineral.

Low Yurisite

Low Yurisite, as the more common form is referred to, is a largely inert stone with texture and weight comparable to ebony. When heated it becomes reasonably malleable, which made it extremely popular for use in construction. During the founding of the Septarchy, it was discovered that in addition to being easy to work with, Yurisite was extremely resistant to artillery bombardment. The shells tended to embed themselves in the walls without doing significant structural damage – much like flinging small rocks at a sand castle. The most famous application of this discovery was the Onyx Fortress, and many argue that this defensive advantage was the biggest reason the Ebon Legion was able to remain independent despite repeated incursions by the Septarchy.

In addition to its uses in construction, popular lore holds that Low Yurisite has healing properties. There has been little conclusive proof to this effect, but it is worth noting that almost all doctors bring a few stones along whenever they are treating a patient.

Despite the best efforts of the Universal Church of Gilded Dagon, however, no method was ever found to refine Low Yurisite into Cloudstone.

High Yurisite

High Yurisite was almost entirely visually dissimilar from Low Yurisite. The crystals were perfectly clear and glow with a soft blue light that emanates from the center of the stone. People staring into that light were often overwhelmed by its beauty and were known to remain enraptured for hours. After the mineral's initial discovery around the fifty year anniversary of the Septarchy (52 SE), it became hugely popular in jewelry. Thirty years later, when the Cloudstone refining process was discovered, its use as an ornament became prohibitively expensive for all but the extremely wealthy. By the rise of Septarch Quintillian, a High Yurisite ring could be sold for more than one hundred times its weight in gold.

High Yurisite was exceedingly rare. Less than thirty percent of all Yurisite deposits contain High Yurisite, and where it was found, yields higher than two percent of recovered crystals were unheard of. Fortunately, or unfortunately – depending on your point of view – new deposits of High Yurisite were consistently found for the entire period leading up to the Sandalphon Affair, allowing its continual refinement and use.

In addition to its rarity, High Yurisite was extremely fragile, further confounding efforts to mine it. Application of even moderate heat or pressure was sufficient to cause the mineral to crack. When this happens, the blue glow within the gem fades, it becomes cloudy and within minutes has transformed into Low Yurisite.

The Refining Process

Cloudstone is a refined version of High Yurisite. The exact details of the refining process were kept a closely guarded secret of the Universal Church of Gilded Dagon, and were largely lost during the final days of the Sandalphon Affair. What remains are fragmentary descriptions recovered from the ruins of the Fallen Basilica, such as the following excerpt from the diary of Pope Sennis:

Earlier that morning, I had learned of the governor's edict declaring a new tax on the sale of barley. The results of this would have been disastrous for the church's farms, so I had resolved to fix the problem. Unfortunately, a refining ceremony was scheduled for the morning and I had no choice but to attend. When I arrived, the stone had already been placed in the Osmotic Device, and the giver had been anointed. I provided the benediction and wrote the final lines of the Eccentric Script, connecting it to the base of the device. While the monks performed the ritual dance and chanted the hymns – I still wonder what language those hymns are in – I devised a way to force the governor to retract his edict. I blessed the giver, and as he was carried from the room, I rushed to implement my strategy...

Cloudstone

Once refined, the stone developed milky white patches that move around within it. The stone still retained its light blue glow, and the combined effect looked a great deal like clouds floating through the sky. It was this image which gave the stone its name. In addition, the refining process imparts Cloudstone with its most useful property: levitation. It was this levitation which made the stone so valuable. To give an idea of scale: a Cloudstone crystal approximately three inches in diameter had more than enough lift to support a lightly armored military vessel. Most commercial ships use crystals with diameters less than an inch.

As the Cloudstone aged, the milky white patches grew larger and large until the entire stone was opaque. This process usually takes several decades, although towards the end of the period in question, the useful life of a Cloudstone grew noticeably shorter. When the stone was completely opaque, it was discarded. Within a few months of becoming opaque, the Cloudstone cracked and the glow within it vanished. At this point, the stone also lost its powers of levitation. Sailors refered to this as the death of the stone, and being in the presence of a dead Cloudstone was considered extremely bad luck. Stories abound of ships that were stranded in remote regions, and thus outside of the reach of the Church's Collectors, when their Cloudstones died. They invariably finish with the crew members of the ship dying in mysterious ways and the ship being haunted by winged ghosts that were often likened to angels or demons. Despite the stories, there are no creditable cases of these Ghost Ships on record.

—Gwydion Vadeki