Glossary: alkali-beryl

Emerald, aquamarine and morganite are the best known varieties of the mineral called beryl. Chemically speaking, it's a beryllium aluminium silicate with formula Be3Al2(SiO3)6. Then there are the alkali metals of which sodium, potassium and lithium are the most common. Should their oxides be present in a beryl (Li2O to 2%, Na2O to 4%, K2O to 2%, Rb2O to 1.3% and Cs2O to 4.6%), then it is called an alkali beryl. Vorobyevite is generally regarded as an alkali beryl.