Prismatic crystals of emerald embedded in their host rock, a gneissic granite from Cambrian deposits along Lake Mjosen some 60 km north of Oslo, Norway. Emerald is a variety of beryl, a beryllium aluminium cyclosilicate of which the crystal system is hexagonal. When pure and flawless, emeralds are the most precious green gemstones known, but they are rarely flawless. Actual width is 110 mm.