QUARTZ: MOTHER OF MANY GEMS
Quartz is a
very common stone among gems, and probably much more common than is generally
known, as the name “quartz” has only been associated with some of the
crystalline varieties. I can’t tell you all of the reasons why the name has
kept a rather low profile, but it appears that many of the names given to
different varieties of quartz were historically given before quartz itself was
defined by composition and crystalline habit. Other reasons may be because it
is not considered an attractive or popular name commercially, and the name
could easily be over used if all or many in the quartz family of stones
contained quartz in their names.
So what gemstones are actually quartz?
The purpose of this article is to explain, clarify, and help readers understand
the relationships of the many gemstones in the quartz family.
By definition, quartz is silicon dioxide
(SiO2). It has a hardness of 7; harder than glass (with a hardness
of about 5 to 5.5). It is a relatively light stone with a density of 2.65 (or
2.65 times heavier than water). Excepting a few rare minerals, every kind of
mineral or stone composed of silicon dioxide is quartz.
Quartz is divided into two general
classes which are further divided into subclasses. The general classes are
Crystalline Quartz and Chalcedony. Crystalline Quartz includes any variety of
quartz with clearly observable hexagonal/pyramidal crystals. Chalcedony
includes all amorphous, micro and cryptocrystalline varieties of quartz.
Crystalline
Quartz Varieties
Crystalline varieties of quartz in the
gem world are transparent to translucent, and are of generally higher value than
the Chalcedony varieties, especially when exhibiting consistent and rich color
saturation, and/or are faceted. They are usually formed under volcanic or
hydrothermal (superheated water) conditions, followed by relatively slow
cooling. The sub classes are defined simply by crystal size.
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| Quartz Crystal at jemelww.com |
In the large crystal class, each stone
type has its own name. Some stones have still retained the quartz name. These
include Quartz Crystal (clear
variety), which is often instead called Rock Crystal or simply just “Crystal”; Rose Quartz (pink variety); Smoky Quartz (grey, brown, or black
variety); Rutilated Quartz, (rutile
crystals within clear quartz); Tourmalinated
Quartz (tourmaline crystals within clear quartz); and Dendritic Quartz (dendritic manganese or other moss-like inclusions
within clear quartz). Also in the large crystal class are Amethyst (lavender to purple variety), and Citrine (golden yellow to yellow-orange variety).
Generally, the small crystal class is
simply called Drusy or Druzy. Originally drusy was defined as
crusts of tiny crystals lining the inside of an agate or rhyolite geode, but
the term has expanded to include many kinds of tiny crystal crusts with no
regard to their origin or kind of formation. This has sometimes led to more
specific distinguishing names such as agate drusy or drusy chalcedony. While
the term drusy alone is commonly understood to mean a fine quartz crystal
crust, because of an increased variety of small crystal crusts, it is often
further clarified by adding a stone name either before or after the term, such
as amethyst drusy or drusy quartz. There are many kinds of crystal crusts
called drusy, including minerals outside the quartz family. Pyrite drusy is one
example. Crystals on a drusy stone typically have a size range from fine to
coarse sand. Drusy stones are valued mostly for their sparkling or shimmering
surface.
Chalcedony
Varieties
Chalcedony varieties of quartz in the gem
world are typically translucent to opaque. They may be formed under volcanic or
hydrothermal (superheated water) conditions, followed by relatively rapid
cooling; or in either sedimentary or metamorphic environments. The subclasses
are Chalcedony, Agate, and Jasper with some few stones somewhat associated with
one of these classes, but nevertheless in classes of their own.
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| Brown Chalcedony Rose at jemelww.com |
The general Chalcedony class includes
most stones that are not further defined by either the agate or jasper classes.
White, Brown, and Black Chalcedony,
and Chalcedony Roses (uniquely
shaped bubbled and rippled free form stones) are part of this class, as well as
Carnelian (orange to orange brown), Blue Chalcedony (light blue to medium
blue-grey), and Chrysoprase (green
to yellow-green). All of these stones are typically translucent to some degree
and often appear to radiate light or have a glowing effect.
There is some confusion in distinguishing
the differences between the Agate and Jasper subclasses in reference to
historical, modern, and commercial definitions (a topic for further
discussion), but agate is usually defined as cryptocrystalline (fine fibrous
crystal structure) quartz, translucent to opaque, with an ordered pattern of
lines, bands, layers, concentric circles, etc. The Agate subclass includes: Botswana Agate (fine concentric lines
of clear, white, brown and grey); Blue
Lace Agate (alternating bands of different shades of light blue); Brazilian Agate (Bands of varying
shades of translucent grey or brown), and a great many others – too many to
list.
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| Bloodstone at jemelww.com |
A few stones generally fall under the
Chalcedony class, but do not quite fit into any of the sub- classes. These
include replacement stones such as Petrified
Wood (ancient wood replaced by quartz – var, jasper) and Tiger’s Eye (fibrous crystals of
crocidolite, riebeckite, or other asbestos like minerals replaced by quartz);
and metamorphic quartz (quartzite) gem called Aventurine (green,
red, white, or orange-coral color with sparkling flecks of tiny mica or
goethite crystals)
Copyright 2015 Steven
A. Barben
General References
Pough, Frederick. 1983. Peterson Field Guides: A Field Guide to Rocks and Minerals.
Houghton
Mifflin Co.
Mottana, Annibale; Crespi, Rodolfo; and Liborio, Giuseppe.
1978. Simon & Shuster’s Guide to Rocks and Minerals. Simon &
Shuster Inc.
Note: Minor sources may include, but are not limited to
a variety of printed and online sources.
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