Quartz is the most common mineral on the face of the Earth. It is found
in nearly every geological environment and is at least a component of almost
every rock type. It frequently is the primary mineral, >98%. It is also
the most varied in terms of varieties, colors and forms. This variety comes
about because of the abundance and widespread distribution of quartz. A
collector could easily have hundreds of quartz specimens and not have two
that are the same due to the many broad categories. The specimens could
be separated by answers to the following questions:
tapered?, coated?, microcrystalline?,
Multiple combinations of these could
produce hundreds of unique possibilities.
Some macrocrystalline (large crystal) varieties are well known
and popular as ornamental stone and as gemstones.
is the purple gemstone variety.
is a yellow to orange gemstone variety that is rare in nature but is often
created by heating Amethyst.
is the cloudy white variety.
is a leek-green gemstone variety that is rare in nature but is created by heating Amethyst from certain locations.
is the clear variety that is also used as a gemstone.
is a pink to reddish pink variety.
is the brown to gray variety.
Cryptocrystalline (crystals too small to be seen even by a microscope)
varieties are also used as semi-precious stones and for ornamental purposes.
These varieties are divided more by character than by color.
The primary varieties of
chalcedony are as follows:
Agate is a banded variety
(sometimes with translucent bands)
green with red speckles
Carnelian is yellow
Flint is generally black
with a fibrous microscopic structure
Jasper is any colorful
Onyx is black, white, or
alternating black and white
Sard is yellow to brown
Sardonyx is banded,
alternating sard and (usually white) onyx
Check out our selections of agate bookends,
dyed blue, and
Quartz is not the only mineral composed of SiO2.
There are no less than eight other known structures that are composed of
SiO2. These other substances and quartz are polymorphs
of silicon dioxide and belong to an informal group called the
Group or Silica Group. All members of this group, except quartz,
are uncommon to extemely rare on the surface of the earth and are stable
only under high temperatures and high pressures or both. These minerals
have their own unique structures although they share the same chemistry,
hence the term polymorph, which means many forms.
Quartz has a unique structure. Actually, there is another mineral that
shares quartz's structure, and it is not even a silicate. It is a rare phosphate
AlPO4, that is isostructural with quartz. The structure
of quartz involves corkscrewing (helix) chains of silicon tetrahedrons.
The corkscrew takes four tetrahedrons in order to repeat itself, or three
turns. Each tetrahedron is essentially rotated 120 degrees. The chains
are aligned along the C axis of the crystal and interconnected to
two other chains at each tetrahedron making quartz a true tectosilicate.
This structure is not like the structure of the chain silicates or inosilicates
whose silicate tetrahedronal chains are not directly connected to each
other. The structure of quartz helps explain many of its physical attributes.
For one, the helix makes three turns and this helps produce the trigonal
symmetry of quartz. Likewise a helix or corkscrew lacks mirror planes of
symmetry as does quartz. The corkscrew structure would also disrupt any
cleavage which requires a plane of weakness not found in quartz and breakage
would result in the curved fracture, conchoidal, that is found in
quartz. Quartz can also have left and right handed crystals just as a corkscrew
can screw in a left handed way or in a right handed way. There are even
some very difficult to identify crystals of quartz that are twinned with
alternating one sixths of the crystal being right handed and then left
Quartz is a fun mineral to collect. Its abundance on the Earth's surface
is incredible and produces some wonderful varieties that don't even look
like the same mineral. A collector must always be up on the many
varieties of quartz and it sometimes embarrasses a collector to have collected
too many specimens of such a common mineral. But nearly all collectors
concede that you can never really have enough quartz specimens. Note that quartz
is nearly a defining component of most geodes.
The agate form typically lines the original cavity, creating a durable shell to
contain subsequent mineral growth, which itself is often quartz crystals of one
or more varieties. Geodes from Brazil are a good example, as layers of agate
comprise the shell, lined with milky and/or colorless quartz crystals, topped
Color is as variable as the spectrum, but clear quartz is by
far the most common color followed by white or cloudy (milky quartz). Purple
(Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are
also common. Cryptocrystalline varieties can be multicolored.
Luster is glassy to vitreous as crystals, while cryptocrystalline
forms are usually waxy to dull but can be vitreous.
Transparency: Crystals are transparent to translucent, cryptocrystalline
forms can be translucent or opaque.
Crystal System is trigonal; 3 2.
Crystal Habits are again widely variable but the most common
habit is hexagonal prisms terminated with a six sided pyramid (actually two
Three of the six sides of the pyramid may dominate
causing the pyramid to be or look three sided. Left and right handed crystals
are possible and identifiable only if minor trigonal pyramidal faces are
present. Druse forms (crystal lined rock with just the pyramids showing)
are also common. Massive forms can be just about any type but common forms
include botryoidal, globular, stalactitic, crusts of agate such as lining
the interior of a geode and many many more.
Cleavage is very weak in three directions (rhombohedral).
Fracture is conchoidal.
Hardness is 7, less in cryptocrystalline forms.
Specific Gravity is 2.65 or less if cryptocrystalline. (average)
Streak is white.
Other Characteristics: Striations on prism faces run perpendicular
to C axis, piezoelectric (see tourmaline)
and index of refraction is 1.55.
Associated Minerals are numerous and varied but here are some
of the more classic associations of quartz (although any list of associated
minerals of quartz is only a partial list): amazonite a variety
Notable Occurrences of amethyst are Brazil, Uraguay, Mexico,
Russia, Thunder Bay area of Canada, and some locallities in the USA. For
Smoky Quartz; Brazil, Colorado, Scotland, Swiss Alps among many others.
Rose Quartz is also wide spread but large quantities come from brazil as
do the only large find of Rose Quartz prisms. Natural citrine is found
with many amethyst deposits but in very rare quantities. Fine examples
of Rock crystal come from Brazil (again), Arkansas, many localities in
Africa, etc. Fine Agates are found in, of course, Brazil, Lake Superior
region, Montana, Mexico and Germany.
Best Field Indicators are first the fact that it is very common
(always assume transparent clear crystals may be quartz), crystal habit,
hardness, striations, good conchoidal fracture and lack of good cleavage.