Sidcup Lapidary & Mineral Society

Home

Contact Us
History
Schedule
Tour of Room 26
Galleries
Links
Archive

Bi-COLOURED MINERALS DISPLAY EVENING

Many minerals or crystals occur in different colours or shades and often some crystals of a single mineral show a grading from richer to paler tints within a single crystal or fragment.

Bi colouring or multicolouring of a single crystal of any particular mineral is not so common but a few minerals are especially prone to this.  
The colouring can often occur as sharp well defined zones and in some cases can help with an insight to the growth of a crystal and its composition
.


Page from George Frederic Kunz's Publication of 1890:
"Gems and Precious stones of North America"
showing illustrations of Tourmaline crystals from Maine, USA.,
three of which show colour zoning.

 

On Monday evening, October 5th '09, members got together on the club evening and brought along a selection of minerals showing this phenomenon.
Here are some of the things that were on display.


Ennis brought in a group of small items which show to a greater or lesser degree colour banding and zoning.

This amethyst from Artigas, Uruguay has been cut and polished both sides.
Notice the colourless base of the quartz crystals which change to purple towards the edge of the piece.
Notice also the banded agate around the basalt core.

 

A polished heart fashioned in Ametrine (Amethyst and Citrine) from Bolivia.

 

Two more British pieces now.
First, a nice twinned green crystal of Fluorite from Weardale Co. Durham,
showing a core of deeper more bluish colour.
A classic example of colour zoning in crystals.



A partially polished pece of the classic fluorite Variety "Blue John" from Castleton, Derbyshire
showing the typical inky blue colour banding.

 

 


Graham brought in one or two cut and polished slices showing many colours and extraordinary patterns.

The first three images are close up of an agate he collected from Wheal Mary Anne, Cornwall.

 

Below is a classic Arizona Fossil Log Slice cut and polished to show the patterning.
Natural wood replaced by silica, preserving the material with many colours.

 

Another multicoloured example.
An Agate from Brazil.
Again, cut and polished to show its features.

 

Graham also had this huge, cut and polished, half nodule of "agate-type" potato stone from around the Dulcote district, Somerset.


Finally, an unusual candidate.
A slice of Connemara marble from Ireland showing shades of pale green and yellowish banding.

 

 


Martin and Jeremy brought in a few items from different parts of the world.

This colourful banded nodule baryte from Wales

 

Cut and polished slab of banded Goethite / Haematite from Merehead, Shepton Mallet, Somerset.

Detail of same:

 

A small half nodule from the Dulcote area again with colour banding.

 

A polished egg fashioned from Tiger Eye.

 

From Jeremy's collection a rather good piece of Chalcedony from Morocco showing a little colour variation.

 

At first glance this might seem a fairly standard piece of Malachite on Chrysocolla from the Kalukuluku mine (Ex: Star of the Congo Mine), albeit a large and well looked after example.

 

Closer inspection shows two growth phases of malachite.
Tiny dark crystals lying on top of paler botryoidal malachite.

 

 


Some other Bi-coloured and multi-coloured minerals:

Green and pink Tourmaline from Afghanistan


Two images of a small group of Tourmaline crystals from The Aracuai District, Minas Gerais, Brazil.

 

Perhaps not immediately obvious from the photo, this quartz crystal from Brandberg, Namibia shows color zoning from completely colourless at the tip to lilac (Amethyst) with Smokey quartz patches.
The small "tufts" along the back edge are prehnite groups.
The whole piece is a bout 60mm long.

 

Perhaps one of the rarest forms of tourmaline from the Paraiba deposit, Brazil.
A rather fragmented piece (far from gemstone quality), the electric blue colour is caused by copper.
Notice the colour change towards the edges to a black "skin". Areas of pink tourmaline are also visible.
The matrix (white material) towards the back is quartz.



The Anjanabanoina pegmatites of Madagascar have produced some of the finest tourmalines ever found, some as very large crystals.
When cut and polished into slices some reveal a complex multicolour internal pattern which is not easily seen from the outside of the original complete crystal. Notice the green "rind".


Detail of above:

 

Another more uniformly coloured "Watermelon" tourmaline crystal slice from an unknown location:

 

Detail of green watermelon "skin" on magenta core:

 

An unusually strong colour zoning in a small Kyanite crystal from Brazil

 

A multi-coloured example now, although this is cheating a little really.
Labradorite Feldspar from Madagascar.
The colours produced as a surface sheen on this mineral are not from "body" colour.
They are produced by the interferance of light caused by ultra thin sheets of another feldspar, only seen when the rock is held at a certain angle.


 

A little closer !

 

More Tourmalines !

Nigerian tourmalines aren't easy to come by as most of them are so good they end up in the cutters hands. This remarkable crystal escaped perhaps because it maybe too pale.

Although small (around 20mm long) and entirely transparent it is colour zoned with a pale pink band in the centre and a very unusual and very thin dark ink blue termination.

Another view

 

Although to a minor degree bicoloured (Each end of this double terminated crystal are a slightly paler greenish hue as opposed to the near black centre section),
the real curiosity of this crystal is its twisted shape.


Another view to show the attached Muscovite and feldspar.

 

A small pink and green tourmaline from Aracuai, Minas Gerais, Brazil.


Two remarkable Tricoloured Tourmaline crystals from the well documented Sapo Mine, Minas Gerais, Brazil.


And finally a selection of small bicoloured crystals from Brazil, all terminated:

 


Home