From Halopedia, the Halo wiki
Boron, chemical symbol B, is a trivalent nonmetallic element that never occurs as a free element in nature. In amorphous form, it is a brown powdery substance, while in crystalline form, it is hard and black. Boron has an atomic number of 5, a hardness (Moh's scale) of 9.3, and belongs to group 13 on the periodic table.
Because of the remarkable hardness of boron compounds, boron fibers are commonly used by the UNSC to reinforce other metals. One example is prior to Operation: FIRST STRIKE, when the Spartan-IIs of Blue Team used boron fibers to reinforce the hull of a captured Spirit dropship.