A rare mineral detected in large deposits inside the huge comet that Enterprise NX-01 stopped to study in Breaking the Ice, prompting Reed and Mayweather to land on the comet and drill for a sample.
Eisilium is a scarce mineral, hard enough to come by that even the Vulcans have had little chance to examine it. It appears in the Enterprise episode Breaking the Ice in 2151.
Enterprise comes upon the largest comet humans have yet seen, and scans show a rich deposit of eisilium some way below the surface. Archer sends Malcolm Reed and Travis Mayweather down by shuttlepod to set charges and extract a core sample; the pair also take the chance to build a snowman. Meanwhile the Vulcan ship Ti'Mur under Captain Vanik arrives and silently watches, to Archer's irritation.
The blasting alters the comet's rotation, bringing the landing site into sunlight sooner than planned. The ice gives way beneath the shuttlepod, and Archer has to swallow his pride and accept Vanik's offer of a tractor beam to pull the men clear.