Beryl
From apm
A hard beryllium aluminum silicate.
Beryllium is rare so usefulness as structural material is limited.
Al2Be3[Si6O18]
- 🟩 Green form of beryl is the widely known emerald.
Related
- Bromellite: BeO, Mohs9, hexagonal
- Topaz … another unusually hard widely known silicate but avoiding elemental scarcity