The Earth Story

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Kunzite, with Tourmaline

The beautiful pink to violet variety of the lithium silicate pyroxene group mineral spodumene is a relatively recent addition to the gem inventory. It was first described in 1902 by the gem and mineral dealer G.F. Kunz (who worked, amongst others, with Tiffany's, the Smithsonian and assembling J.P. Morgan's world class mineral collection) after its initial discovery in California famous Pala district pegmatites. Crystal surfaces are often covered in triangular arrowhead shaped etch pits where the crystal was dissolved by hydrothermal fluids during its residence in the mother rock.

The mineral is strongly pleochroic, displaying three different colours depending on the orientation from which it is viewed. This is due to the different pathways rays of light take within the crystal when they are refracted and split into 3, resulting in different absorptions and residual colours visible to the eye. As a result, gem cutters have to be sure to cut it in the correct orientation to display the deeper violetish hue through the top table facet rather than the pale pink to colourless one. Though it has decent hardness (6.5-7 on Mohs scale), faceting is complicated by the mineral's perfect easy cleavage, where the crystal splits along lines of weakness with fewer atomic bonds in the crystal structure.

The best examples come from the pegmatite fields of Nuristan in Afghanistan, and display strong violet, pale pink and pale green shades. Other localities include Madagascar, Brazil and the USA. Most kunzites are quite pale, and more saturated shades (due to substitution impurities of manganese in the crystal structure) are quite rare. It is one of the few gems available in large sizes relatively inclusion free at reasonable cost. Quality depends on depth of colour and clarity, and the larger the stone, the deeper the colour. Crystals in the Black HIlls of Dakota have been found up to 14.3 metres long. The largest cut gem weighs 880 carats and is held by the Smithsonian Institution.

The colour can slowly fade in direct sunlight, so wearing it for prolonged periods under Sol's rays is not advised. Some specimens have been heated or irradiated to enhance the colour. In this amazing 9.7 x 3.7 x 3.0 cm specimen from Nuristan mined in the 1970's in Afghanistan the somewhat ordinary oblong of Spodumene has a series of gemmy pink Tourmaline crystals growing out of it in a beautiful steamboat fan.

Loz

Image credit: Joe Budd/Rob Lavinsky/iRocks.com

http://www.mindat.org/min-2289.html http://www.gemstone.org/index.php… http://www.minerals.net/gemstone/kunzite_gemstone.aspx http://www.gemologyonline.com/kunzite.html_ _

Pleochroic tanzanite - changes color as it is turned

What a hue!

We recently shared another lovely crystal of Tanzanite (see http://on.fb.me/1B8IMQy for my original post on this mineral) that displayed the multiple colours of its pleochroism, the property some crystals have of displaying multiple colours from different angles (see http://on.fb.me/1DWqeoo). In this image, the single, most perfect shade has come to the fore, in a 8cm long crystal that was nicknamed 'The Deep Blue'.

Loz

Image credit: Malte Sickinger

One crystal, two colours, no magic...

The beauty of gems and crystals is an interaction between mind, mineral and light, and some minerals have structures that produce interesting optical effects, opal being an obvious example. The beautiful tanzanite crystal in the photo (see http://on.fb.me/1B8IMQy for an intro to this purple wonder) is exhibiting another of these properties, which goes under the name of pleochroism, a word coming from the Greek for many colours. A pleochroic crystal actually appears different colours depending on the angle of viewing, and some gems have a very mild version, while others, including tanzanite, a somewhat more extreme one.

Crystals are regular arrangements of atoms in a symmetrical repeating lattice, and while the number of individual patterns atoms can order themselves into is almost infinite, all minerals crystallise in one of 6 (or 7 depending how you divide things) crystal systems, based on the length of the axes of their unit cell (the smallest 'chunk' of a mineral that can exist) and their angles of intersection. One system is regular and even, known as cubic or isotropic, with equal length axes meeting at right angles, the others exhibit ever greater degrees of wonkiness. This distortion is what allows the gem to play with light in this way and delight our eyes.

As light enters the crystal and encounters the lattice, it flows through it down different pathways following the axes, with the rays splitting into two or three. Some directions will be more densely packed with atoms than others, and the light will move at different speeds through them, and be absorbed differently. Since the colour that we see is what is left behind when the crystal has selectively absorbed some of the wavelengths of the light passing through it, different colours can result in the different rays, which then become apparent when the crystal is rotated, or, as in this case, when a change in growth direction occurred.

Each pathway through the lattice also polarises the light, forcing it to vibrate in a single direction, and this property is the basis of the gadget gemmologists use to see this phenomenon. Two chunks of Polaroid plastic, orientated at right angles to each other are set in a circle and used to look at the gem. Each half of the Polaroid will show one colour. The property is also useful for distinguishing minerals in thin sections, those slices of rock beloved of geologists.

Loz.

Image credit: MIM Museum. http://www.allaboutgemstones.com/gemstone_pleochroism.html http://bit.ly/1CK45Jm http://webmineral.com/help/Pleochroism.shtml#.VSvMq5Om0bg http://www.galleries.com/minerals/property/pleochro.htm7