Iceburst stone

Characteristics
The caverns found beneath the factory city were once volcanic calderas. As temperatures beneath the earth decreased as volcanic activity ceased, the natural gases within the caverns solidified at high pressures, becoming like icy stones. When exposed to temperatures greater than the freezing temperatures in the caverns where the stones are found, the stones rapidly vaporize. This creates a small, bursting explosion from which these stones get their name.

Applications
Due to the practicality of such amounts of gas coming naturally in compact, solid forms, the iceburst stones are used as the source of gas which powers vertical maneuvering equipment. They are placed into canisters made from iron bamboo for storage, and when heated they release the gas necessary to make proper use of the equipment. However, improper treatment or use of the gas yielded from iceburst stones can be fatal.

Trivia
The description of Iceburst stone and its physical formation bear a striking resemblance to, also known as methane ice.