The beryl species is one of the most important in the gemstone world. Two varieties of beryl - the deep green emerald, and the blue aquamarine - are among the most famous of all gem varieties.
All beryl is aluminum beryllium silicate by chemical composition. Beryl is highly regarded for its very good hardness - 7.5 to 8 on the Mohs scale - and its lack of cleavage. It does not have a notably high refractive index, with a rating of 1.562 to 1.602, between that of quartz and topaz. Yet many of the beryl varieties have excellent clarity and transparency, so they make very attractive gemstones. Interestingly, the most valuable beryl - emerald - tends to be the least transparent and most included. However, it makes up for those shortcomings with a colour that is unique in the gemstone world.
Colourless precious beryl is known as goshenite. It was named after the very small town of Goshen in Western Massachusetts where it was first described. Goshenite is found in quite a few beryl deposits around the world, so it is perhaps the most common and least expensive form of beryl.