Getting to know crystals
Nearly all minerals are crystalline, but quite often the individual crystals cannot be detected. They are close-packed and deformed because during crystallisation each crystal was squeezed against its neighbour and could not grow to its ideal crystal shape.
A few minerals have the ability to grow into well-shaped crystals even within a rock mass, but many beautiful individual crystals, such as quartz crystals, can only form within rock cavities where they have had adequate space to grow.
Rarely, certain naturally-occurring impurities give crystals an attractive colour, and the most perfect of these coloured crystals are used as precious and semi-precious gemstones. Some stones - for example agates - can be artificially coloured by means of dyes which are absorbed into the stone. Other crystals have colour artificially induced by heat or radiation.
Some different varieties of quartz are discussed here and illustrated on Plate 3. A few other mineral crystals are shown on Plate 4. The quartz crystals, the rose quartz and the pyrite were purchased from the Ardkinglas Woodland shop at Cairndow. The common quartz, the tourmaline and the garnet were found in pebbles on the shore at Strachur.
3 QUARTZ IN VARIOUS FORMS
A. Common quartz B. Crystal of pure
quartz, or ‘rock crystal’ C. Amethyst D. Citrine E. and F.
‘Rainbow quartz’ G. Rose quartz
Sample 3A (Plate 3, sample A) is an example of common quartz which is crystalline (the crystals would be distinguishable under a microscope) but no single crystals are obvious to the naked eye.
The colourless crystal (Sample 3B) is pure quartz exhibiting its characteristic 6-sided crystal shape with a pyramid at one end. Sometimes pure quartz crystals like this are called rock crystal.
Amethyst (Sample 3C) is a well-known purple variety of quartz which occurs quite widely throughout the world but the colour is often rather weak. It is common practice for jewellers and gemstone suppliers to enhance the natural colour using heat and/or radiation. Good quality amethyst is a popular semi-precious gemstone.
Citrine (Sample 3D) is a yellow to brown colour, and may have been formed by the heating of amethyst, either naturally or artificially. Compare this crystal with the natural citrine colour of the quartz in the sample of mica in Plate 1, sample E.
Rainbow quartz (Samples 3E, 3F) is made from colourless quartz crystals which have been deliberately fractured throughout to allow the penetration of different colours of dye.
Rose quartz (Sample 3G) is a form of quartz which has an natural pink colour, although it is rarely found as crystals. It is widely used as a semi-precious stone.
Not represented here is cairngorm, which is usually a smoky colour. Naturally-occurring cairngorm crystals used to be found in the Cairngorm Mountains, and were traditionally used to ornament dirk handles and plaid brooches.
CRYSTALS OF OTHER MINERALS
A. Pebble of common quartz containing
fine needles of black tourmaline B. Pyrite crystals in characteristic
C. Pyrite crystals in simple cubes within slate D. A
band of garnet crystals.
Tourmaline is a much less common mineral than quartz. Usually it occurs as shiny black, well-formed needle-like crystals. Sample 4A, which shows needles of tourmaline in a piece of quartz, was found on Strachur beach. Rarely, tourmaline is sufficiently transparent and delicately coloured to be a sought-after gem stone.
Pyrite has several different crystal forms. In sample of slate from Seil Island (sample 4B) it is in simple cubes. Next to it is a well-crystallised pyrite specimen which exhibits another crystal form which is characteristic of pyrite. This specimen was purchased in local shops and probably came from Mexico! Note the pyrite crystals on the shore south of Creggans Inn, illustrated on page 10. On the other hand, the pyrite which occurs in the samples of gold and zinc ore (Plate 9) is often too fine-grained to be able to identify crystal form at all.
In sample 4D, each of the reddish grains in the middle layer is a single garnet crystal. Large, well-coloured garnets are popular as semi-precious stones for jewellery. This rock sample is from Strachur beach.