Seldum

Seldum is a fictional chemical element that appears in science fiction and tabletop role-playing settings. It is described as a dense, silvery metal with extraordinary stability and electrical properties, often assigned the symbol Sd in-universe and placed among the heavier elements.

Physical properties commonly cited include very high density, high melting point, and resistance to corrosion due to a protective oxide layer. It is frequently said to conduct electricity with low loss and, at cryogenic temperatures, to exhibit superconductivity.

In most depictions, seldum occurs only in trace amounts in ore deposits on distant worlds or in asteroid belts. Mining is portrayed as challenging because it forms compact nodules and requires specialized processing.

Refined seldum is described as forming a versatile base for alloys used in space habitats, superconducting magnets, and high-efficiency energy storage. Some stories also imagine it as a catalyst with unusual surface chemistry.

The name seldum is typically presented as deriving from a fictional language or star-name, reinforcing its status as a distant resource in the lore.

Seldum has appeared in various media, reinforcing its reputation as a rare, valuable material for advanced technology.