Emerald is the green gem variety of beryl, a cyclosilicate with the ideal formula Be3Al2Si6O18. The green comes from chromium, vanadium, or both. Pale green beryl that lacks those coloring agents is usually called green beryl, not emerald.
Beryl is hexagonal, hardness about 7.5 to 8, and often forms six-sided prisms. Natural emerald is typically included and fractured. Those inclusions do not change the mineral species. Aquamarine is the blue variety of the same species and is not covered on this page.