Chrysoberyl is a mixed oxide mineral of beryllium and aluminum (BeAl₂O₄). It is found in beryllium-rich pegmatite dikes; Brazil is a leading source. Its hard, hexagonal system crystals are prismatic or tabular. It can be transparent, green, yellow, or brown, but bright yellow-green is most highly valued. Alexandrite and cat's eye are gem varieties. Hardness 8.5, relative density 3.69.