Quartz
Quartz is somewhere on the list of top-five most abundant minerals, and thereby known
from scores of deposits, habits, varieties, and associations. We can only point out a few
salient features, and encourage you to consult the references below for more
comprehensive coverage. Quartz has been known since antiquity, and origin of the name
seems lost.
Macro crystalline Quartz
Coarse crystalline Quartz is an important component of rocks like granite and pegmatite,
and in different varieties such as Rock Crystal, Smoky Quartz, Amethyst, Citrine, and
Rose Quartz it is important as lapidary material and specimens. The references below
will likely tell you more about Quartz, than you will ever want to know, but allow us to
point out one grave error copied in most text books and identification guides covering
Quartz: Practically all insist Quartz does not have a cleavage. Actually Quartz has
anywhere from poor to good, interrupted cleavage parallel to the rhombohedron (the
'pyramidal' termination), and several others less obvious. Do check Flšrke at al. (1981)
and Thomaz & Barbosa (1902) in the bibliography, and particularly references in the
first. Very often you are able to observe that cleavage yourself in a piece of massive
Quartz (f.ex. Rose Quartz). Here and there you will see straight 'lines' indicting cleavage
planes, often there will be a swarm of parallel planes, and sometimes two or three
swarms of parallel planes, the planes intersecting at the angles corresponding to the
termination on Quartz crystals. Not only does Quartz have several cleavages, they have
been known and repeatedly described for over 150 years, and still don't make their way
into text books!
Cryptocrystalline Quartz
Cryptocrystalline Quartz, with or without components of hydrated, amorphous silica
are important lapidary materials, including a host of Agates, Chalcedonies, and Jaspers -
note, that many stones sold for 'Agate' are not Agate, but some other siliceous rock.
Chrysoprase is a vividly green Chalcedony, the fabled 'blue lace Agate' from South
Africa is a banded, blue Chalcedony, Tiger-eye is a silicified and altered asbestos rock,
etc.
Smoky Quartz
Dark grey or black Quartz is called 'Smoky Quartz'. The colour is due to so-called
lattice defects - imperfections in the crystal lattice. This is a different colouring
mechanism than seen in for example Jasper (microscopic inclusions of other minerals),
and Azurite (colour is an inherent property of the mineral).
The lattice defects of Smoky Quartz may be caused by radiation from the surroundings.
If the host rock contains radioactive minerals, the radiation may alter an otherwise
colourless Quartz to become smoky. This is a physical change of the properties of the
Quartz - some links in the crystal lattice are broken - and it does not make the Quartz
radioactive.
Smoky Quartz is far less common than normal, white or clear Quartz, and good
specimens command a very high price. Most of the Smoky Quartz sold for collectors
and for souvenirs has been irradiated artificially. You can mimic the lattice defects in
natural Smoky Quartz by exposing it to gamma radiation. The same physical process
takes place, it is merely a matter of whether it is in the ground or in front of a gamma
source.
Most natural Smoky Quartz comes from pegmatites or granites. These rocks frequently
carry the minerals, providing the radiation to change the colour of the Quartz. The
Smoky Quartz may be found as grains in the host rock or as free standing crystals in
pockets and cavities. It is the latter - and by far rarest - mode of occurrence, that
produces aesthetic specimens for collections, and high grade cutting material.
Just like any other Quartz, Smoky Quartz is usually cracked or included. Good quality,
transparent, completely natural Smoky Quartz crystals are very rare. The original
crystal has to be perfect & flawless, it has to grow in a suitable environment, and then be
collected with the utmost care. Only very few specimens match all the requirements.
Rose Quartz
Quartz with a distinct pink or reddish tone is called 'Rose Quartz', but the colour and
properties vary considerably, depending on the source of the material. The colour is
caused by microscopic inclusions of other minerals, and there is consequently a trade off
between colour intensity and transparency. A dark piece has many inclusions, and tends
to be less transparent than a paler piece. Perfectly transparent Rose Quartz does not
exist, and reasonably clear pieces are quite rare.
Rose Quartz usually occurs in the core of pegmatites, but most of the Quartz in these
pegmatites is white or colourless, nevertheless. If you mine Rose Quartz, you throw
away more than you preserve! There is usually a gradual transition between white and
pink portions of the Quartz. Consequently, it is comparatively difficult to get large
pieces of Rose Quartz of uniform colour. Most pieces show variation of colour or
occasional white or grey bands. Note that the colour of Rose Quartz depends on the light
source. Most Rose Quartz looks wonderful in sun light or incandescent light, but
absolutely horrible in fluorescent light [and fluorescent light tubes are very common in
shops and exhibition halls!]. Frequently it looks paler, but more pink [rather than
purplish or brown] in sunlight than in incandescent light. Some Rose Quartz fades, when
exposed to strong sunlight for long periods of time.
It is very labour intensive to quarry Rose Quartz. All of the material has to be quarried
manually. If you use explosives, the Quartz shatters and is filled with minute cracks. It is
cheaper to use explosives, but it also produces less good material than manual quarrying.
All of our Rose Quartz is quarried manually. The workers identify a promising area,
drill 1.5-3 m long holes approximately 15 cm apart, and insert chisels or wedges into the
holes. They drive the wedges deeper into the holes, beating once on each wedge at a time
with a sledge hammer, repeating the process until the rock cracks. This rough material
is then divided into smaller blocks and sorted into different grades. Even a good
operation produces less than 20% salable material, and over 80% is discarded.
Rose Quartz occurs at least sporadically in all areas with pegmatites - including
Germany, Scandinavia, and USA, but due to the intensive labour requirements, it is
usually produced in countries with low labour costs. The chief producers are located in
Southern Africa and in South America.
Amethyst & Citrine
Purple coarse crystalline Quartz is called Amethyst. It is cherished as a gem, lapidary
material, ornamental rock, and for mineral specimens. Most Amethyst was formed by
hydrothermal processes (deposited by water at comparatively low temperatures) in voids
in basalt (volcanic rock). The gorgeous Brasilian druses line vugs in basalt, that may be
as much as two meter long, and the intricate prismatic crystals from 'Brandberg' in
Namibia grow individually in basalt vugs as well.
There is some dispute about the origin of colour in Amethyst, but it seems to be due at
least in part to iron in colour centers. When heated, Amethyst changes colour, first it
turns brown and becomes 'Citrine', subsequently it looses colour and turns grey or
white. The transformation is not reversible.
Yellow to golden brown, coarse crystalline Quartz is called Citrine. Virtually all
commercially available Citrine is produced by heat treatment of Amethyst. The only
prominent exception from this is the Bolivian 'Ametrine', that is naturally occurring
intergrown Amethyst and Citrine.
Specimen Handling
Quartz is for all practical purposes stable in a normal household environment. It is not
harmed by light, changes in temperature in the normal comfort range, or known to
decompose. Quartz specimens can be fragile and should be handled with care like any
other mineral specimen. Quartz is not appreciably soluble in water. But ... you may wish
to watch out for the following:
Many Quartz crystals, including Amethyst, etc., contain voids filled with water
and gas, as do so-called 'enhydros', Agate geodes enclosing a large cavity filled by air
and water. Over time the water may evaporate, leaving just a void. You may believe it is
'sealed in,' but take my word for it, it isn't. Water vapor easily passes through Agate and
Quartz. Such specimens may also be susceptible to damage if exposed to high
temperatures or sun light. Keep them in the shade, and don't cry when the water is gone.
Some Amethyst and some Rose Quartz is susceptible to fade, when exposed to sun
light. The rule of thumb is, all Amethyst fades, but most of it fades slowly, and Rose
Quartz coloured due to phosphate (chiefly Rose Quartz crystals and material from a few
Brasilian localities) fades rapidly, but Rose Quartz coloured by Rutile does not fade.
Many brightly coloured Agates are dyed, and some dyes fade. Blame that on the
processor, not the rock! Some natural colour Agate may fade too, though.
Bibliography
Anthony, John Williams, Richard A. Bideaux, Kenneth W. Bladh & Monte C.
Nichols. 1995. Handbook of mineralogy, vol. 2.2
Bancroft, Peter. 1984. Gem & Crystal Treasures.
Bauer, Max. 1896. Edelsteinkunde, 1st ed.
Bauer, Max. 1909. Edelsteinkunde, 2nd ed.
Bauer, Max. 1968. Precious Stones, I-II. Dover Publications
Blackburn, William H. & William H. Dennen. 1997. Encyclopedia of mineral
names. Canadian Mineralogist, special publication 1.
Dake, H.C., Frank L. Fleener & Ben Hur Wilson. 1938. Quartz Family
Minerals.
Epstein, David Stanley. 1988. Amethyst from Brazil. Gems and Gemology, 24(4),
214-228.
Flšrke, Otto W., Heinz G. Mielke, Jürgen Weichert & Holger Kulke. 1981.
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66(5-6), 596-600
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ed.
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[reprinted by D.W. Berger 1990s, n.d.]
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250.
Noe-Nygaard, Arne. 1966. Mineralogi, 3rd ed.
Ramdohr, Paul & Hugo Strunz. 1980. Klockmann's Lehrbuch der Mineralogie,
16th ed.
Roberts, Willard Lincoln, Thomas J. Campbell & George Robert Rapp jr. 1990.
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Rutland, E.H. 1963. Corundum and amethyst from Tanganyika. Journal of
Gemmology, 9(4), 132-135.
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Sinkankas, John. 1964. Mineralogy.
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Anah’ Ametrine Mine, Bolivia. Gems & Gemology 30(1), 4-23
This page is authored by Claus Hedegaard.
