In the 19th century the preferred colour for aquamarine was sea-green, and indeed the name itself means sea water. Today the most valued colours are sky-blue and dark blue. Aquamarine is dichroic, appearing blue or colourless as the stone is view from different angles. Gem-quality aquamarine is found as hexagonal crystals, which may be up to 1m(39in) long and flawless, with striations along the length of the crystal. Aquamarine is often cut with the table facet parallel to the length of the crystal in order to emphasize the deepest coloration.

Crystal Structure Hexagonal
Composition Beryllium Silicate
Luster Vitreous
Hardness 7.5
Refractive Index 1.57 to 1.58
Specific Gravity 2.69
Mohs Hardness 7.5 to 8.0
DR 0.006