Quartz is somewhere on the list of top-five most abundant minerals, and thereby known
from scores of deposits, habits, varieties, and associations. I can only point out a few
salient features, and encourage you to consult the references below for more
comprehensive coverage.
The 'Quartz group' includes minerals composed of silicon dioxide with a couple
of close friends with aberrant chemistry. Opal, for example, incorporates water, and
Melanophlogite incorporates organic material, that seems to stabilize the crystalline
structure - the function of this is not entirely understood. Quartz is an important
constituent of most sand, gravel and rocks like granite. Some of the less conspicuous
Quartz group minerals occur in volcanic rocks (Cristobalite, Tridymite), sand fused by
lightening (Lechatelierite), and meteor impactites (Stishovite and Coesite). 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.
Cryptocrystalline Quartz, with or without components of hydrated, amorphous
silica (see Opal below) 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., etc.
Opal is the main skeletal component of living siliceous sponges and algae
(diatoms), and fossil diatoms may form a rock called diatomite. Opal is one of these
minerals that happen to be a mineral because of grandfather status - it was known and
described before anybody got a clear idea of how to define minerals. The substance we
identify as Opal be either hydrated amorphous silica [most text books describe Opal as
exactly, and only, that - hydrated amorphous silica], Cristobalite, Tridymite, or a
combination of these. The play of colours in precious Opal is due to the morphological
organisation of the minerals in small, closely packed spherules [Ramdohr & Strunz
(1980), p. 528 has a beautiful picture of this].
Specimen Handling
Quartz group minerals are for all practical purposes stable in a normal household
environment. They are not harmed by light, changes in temperature in the normal
comfort range, or known to decompose. Quartz group specimens can be fragile and
should be handled with care like any other mineral specimen. Quartz group minerals are
not appreciably soluble in water. But ... this is a group of minerals, and all sorts of
exceptions apply. You may wish to watch out for the following:
Many Quartz crystals, including Amethyst, etc., contain small 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.
The play of colours in precious Opal is due to the intergrowth of different
crystalline and amorphous phases and to water. Consequently, the colours are often lost
when the Opal 'dries out', which may be promoted, delayed, or even prevented,
depending on storage conditions. Common Opal is subject to the same process, but the
visual impact is rarely as profound, though specimens may literally crack.
Bibliography
Anthony, John Williams, Richard A. Bideaux, Kenneth W. Bladh & Monte C.
Nichols. 1995. Handbook of mineralogy, vol. 2.2
Bedemar, Vargas. 1822. Der Opal auf den Faröern. Taschenbuch für die
gesammte Mineralogie von Karl Cäsar Leonhard. 11-30
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
Downing, Paul B.. 1996. African Opal unearthed. After decades of rumors, a
recent find may make Ethiopia an important source of Opal. Lapidary Journal.
1996(7), 39-42
Epstein, David Stanley. 1988. Amethyst from Brazil. Gems and Gemology, 24(4),
214-228.
Flörke, Otto W., Heinz G. Mielke, Jürgen Weichert & Holger Kulke. 1981.
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66(5-6), 596-600
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