Molybdenum

Molybdenum (from the Greek word meaning "lead-like"), chemical symbol Mo, is a gray, metallic transition metal. It has an atomic number of 42, and belongs to group 6 on the periodic table. Molybdenum is found in trace amounts in some plants and animals.

Usage
Molybdenum has the sixth-highest melting point of any element, and for this reason is commonly used by the UNSC in high-strength alloys. One example is prior to Operation: FIRST STRIKE, when the Spartan-IIs of Blue Team used carbon-molybdenum I-beams to reinforce the hull of a captured Spirit dropship.