Sidcup Lapidary & Mineral Society
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.
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.
from George Frederic Kunz's Publication of 1890:
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.
amethyst from Artigas, Uruguay has been cut and polished both sides.
A polished heart fashioned in Ametrine (Amethyst and Citrine) from Bolivia.
more British pieces now.
A partially polished
pece of the classic fluorite Variety "Blue John" from Castleton,
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.
Graham also had this huge, cut and polished, half nodule of "agate-type" potato stone from around the Dulcote district, Somerset.
an unusual candidate.
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.
inspection shows two growth phases of malachite.
Green and pink Tourmaline from Afghanistan
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.
one of the rarest forms of tourmaline from the Paraiba deposit, Brazil.
Anjanabanoina pegmatites of Madagascar have produced some of the finest
tourmalines ever found, some as very large crystals.
Detail of above:
Another more uniformly coloured "Watermelon" tourmaline crystal slice from an unknown location:
of green watermelon "skin" on magenta core:
An unusually strong colour zoning in a small Kyanite crystal from Brazil
multi-coloured example now, although this is cheating a little really.
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.
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
Another view to show the attached Muscovite and feldspar.
small pink and green tourmaline from Aracuai, Minas Gerais, Brazil.
Two remarkable Tricoloured Tourmaline crystals from the well documented Sapo Mine, Minas Gerais, Brazil.