Since Frank's 1952 proposal that supercooled liquids exhibit icosahedral local order, the icosahedron has become central to understanding glass formation, nucleation, and metallic alloys. This paper collects the key geometric properties of the icosahedron relevant to condensed matter physics: the invariants V = 12, E = 30, F = 20; the derived quantities 41 = (V−1) + E and 42 = V + E; the structure ratio e⁵ = e^(E/D!) where D! = 6 is the dimensional factorial; and the fundamental constraint that icosahedra cannot tile three-dimensional space. These geometric facts, combined with Klein's 1884 analysis of icosahedral symmetry, provide a unified foundation for understanding geometric frustration in materials. We present this as a reference for companion papers on the glass transition and crystal nucleation.