mineralogue:

Beryl is a cyclosilicate mineral that comes in a number of gem varieties. It has the chemical formula Be₃Al₂(SiO₃)₆ and is one of my favourite minerals.

Beryl has a hardness of 7.5-8, hard enough to make it an ideal gem mineral. In its pure form, beryl is colourless; however, impurities lend beryl a variety of different colours. The gem varieties of beryl and the ions that give them their colour are listed below.

• ·         Heliodor’s golden-yellow colour derives from Fe³⁺ ions
• ·         Aquamarine’s pale blue colour arises from the presence of Fe²⁺ ions
• ·         A dark blue variety called maxixe receives its colour from the presence of both Fe²⁺ and Fe³⁺ ions. This colour is light-sensitive, fading to white when exposed to sunlight or heat for extended periods. Irradiating faded maxixe will return the colour; green, yellow, or pink beryl can be tinted the dark blue of maxixe through irradiation as well. It was named for the Maxixe Mine in Brazil.
• ·         Emerald’s green colour comes from Cr³⁺ ions. Emerald is highly valued, not just for its rarity but because when it does occur, it is often highly included, increasing the value of good crystals. Vanadium ions may also contribute to emerald’s colour.
• ·         Morganite’s rose-pink colour comes from the presence of Mn²⁺ ions. Orange morganites may be found as well.
• ·         Red beryl, (also known as red emerald or bixbite) is coloured due to Mn³⁺ ions, and is extremely rare. In contrast to other gem varieties of beryl, which are usually found in granitic pegmatites, red beryl is generally found in topaz-bearing rhyolites and form under low pressure and high temperature.

In addition, a colourless variety of beryl called goshenite is found in nearly all beryl localities. While pure beryl is colourless, not all goshenite is pure, since some impurities prevent the expression of any colour that other ions might normally cause. Goshenite used to be used in eyeglasses and lenses. It is not highly valued as a gemstone, but it can be irradiated to produce other, more favoured, coloured beryl varieties.

Beryl does not fluoresce; however, some materials used to fill fractures in emeralds do. Coloured beryls are dichroic.

Sources:

• Wikipedia
• Gemdat
• Webmineral

Image 1: Aquamarine with muscovite mica from Shengus, Baltistan, Pakistan. Sourced from irocks.com 
Image 2: Emeralds from Cosquez Mine, Colombia. irocks.com