The Forbidden Gel
Long ago, the Empire engineered live gel, a substance similar to the Living Tar. At some point, though, the gel was considered to be too dangerous, and the Empire's powerful Tar Authority didn't want competition, so they forbade the use of it.
Much later on though, the gel started appearing again. Several accounts have suggested that not only has the Empire started using gel again, but some on the surface have as well.
This green substance is much stickier and more viscous than its tar and mud counterparts, and because of that, it is only cuttable on its inside corners. Additionally, gel has two unique major differences from tar and mud, as a result of its high viscosity. The first major difference is that its babies, being extra sticky, follow the movement logic of wubbas and rock golems, unlike tar and mud babies, which use roach logic. The other major difference is that gel mothers, with their ability to expand their influence hindered by the gel's increased tension, can only expand masses of gel they're contained in, as opposed to tar and mud mothers, which grow all tar or mud in the room.
See also: General element rules | On the (Ab)Use of Tarstuff | The Living Tar | The Awakened Mud | The Forbidden Gel | Tar and Mud Rivalry