Posts tagged: geology
Emerald crystals on Biotite schist matrix
Leckbachgraben (Leckbachrinne), Nasenkopf, Habach Valley, Hohe Tauern, Salzburg, Austria
Aquamarine on Muscovite - Chumar Bakhoor, Hunza Valley, Gilgit District, Gilgit-Baltistan, Pakistan
PYROLUSITE (Manganese Dioxide) from Imini Mine, Quarzazate Province, Morocco. Photo shows opened pyrolusite nodule with acicular crystal habit at the edges of the nodule.
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.