The Seed Bismuth is an object with unknown properties associated with the creation of Gunnerkrigg Court. Jones explains what little is known about it in a simulation in Chapter 23, although she also says it is not known exactly what it was.
It seems there is a saying, regarding its relevance to the creation of the court, "It grew from the Seed Bismuth." This has been stated by, among others, Old Robot. However, Kat claims when asked how the court was built, everyone at the court just answers that the founders built it. Katerina also speculated that it might actually have grown, like a plant, or like a Tic-toc.
Beesmuth is a wisp, an etheric bee Annie encountered in Chapter 44. It claimed to be the Seed Bismuth. The being told Annie some of the Seed Bismuths story and how much of a part it plays in the story of the Court.
However, as Beesmuth was proven not to be the Seed Bismuth it is unclear if this information is to be taken seriously.
The story Beesmuth told Annie is as follows: The Seed Bismuth was created by magic of the forest animals, technology of the humans , and the nature of the forest. It was then planted into the ground in order to make a place both humans and etheric beings could live, but the humans somehow made Seed Bismuth grow even faster, and so it started overtaking the forest. This resulted in fighting between the two factions. In order to stop this fighting, Coyote created the divide between the forest and the Court, leaving the seed and some of the buildings on the forests side.
After this, Coyote.
This storyline is currently ongoing.