is a gem? Although you might not be able to give a precise definition
of a gem at this time, few of you would have any trouble in recognizing
that the images below are of gems. So then, what characteristics do
they exhibit that allow you to intuitively recognize them, and cause gemologists or geologists
to officially label them as such?
Cabochon and Carved gems
A gem is a natural, mineral or organic substance, that has substantial
beauty, rarity, and durability. Let's take each underlined part of that
definition and examine it.
Natural means that the material was not made, or assisted in its making,
by human effort. When such is the case, modifiers such as "laboratory
grown", "synthetic", "cultured", or "man-made", must, by Federal Trade
Commission (FTC) regulations, be used in the descriptions of any such
pieces being advertised or marketed. Man-made "gems" have all the chemical,
optical and physical characteristics of the natural materials they imitate,
but they do not have their rarity or value. You can be certain whenever
you see any of the above modifiers that the material in question is
not of natural origin.
A mineral can be defined as a crystalline solid with a specific chemical
formula, and a regular three dimensional arrangement of atoms. (In a
later web lecture, this definition will be broadened to include "amorphous"
materials which have a specific chemical formula but do not have a specific
crystalline structure, for example, opal and natural types of glass).
Mineral Gems : Iolite, which has a specific chemical formula of: Mg2Al4Si5O18
and a regular arrangement of atoms which places it into a crystal system,
with other minerals of similar structure, known as the orthorhombic
crystal system is a mineral gem. Another example is emerald, Be3Al2(SiO3)6,
a member of the hexagonal crystal system. (The attributes of
the various crystal systems will be presented in an upcoming lesson.)
Uncut Emerald Crystal
Organic Gems : An
organic gem is one that was made by living things, present or past.
Examples include pearls, coral, jet, ivory, shell and amber. Such gems
consist of the molecules formed by the organism, although these molecules
may have beem altered somewhat due to compression or other geological
or chemical forces.
Coral and freshwater Cultured Pearl earrings
Faceted Amber (enlargement showing fossilized insect within
Gems such as "petrified dinosaur bone" and many other "stony" fossil
gems, are classified as mineral, rather than organic. Although its true
that bone is an organic material: the reasoning involved is that the
original organic molecules and structures of long ago have been totally
replaced with mineral solutions such as silica. (This common geological
process is called petrifaction).
classed as organic gems
Dinosaur bone Agate
cut from a Fossilized Coral Colony
Although none of the molecules from the living organisms remain in certain
types of organic gems, such as the calcareous corals, the minerals they
are composed of were secreted, originally, by the living things as they
grew, not replaced later by petrifaction. Likewise, although substantial
geologic changes have altered the properties of jet and amber, the materials
still consist primarily of the original organic molecules.
Calcareous "angel skin" Coral carved beads
Carved Jet earrings
Circa 1925 Amber and Jet cigarette holder
A gem is beautiful. Beauty, of course, is a subjective concept
that has many aspects, and differs from viewer to viewer, but in general,
the attributes of gems which excite our sense of beauty include, color,
transparency, luster, brilliance, pattern, optical phenomena and, in some
cases, distinctive inclusions.
Kunzite : color, transparency and brilliance
Jasper: color, pattern and luster
Ammolite: color, luster and iridescence (an optical phenomenon)
Rutilated Quartz: transparency and distinctive inclusions
A gem is rare. There are two types of rarity involved : relative
Relative: Many gem minerals occur in various locales and, often,
in large deposits, but the vast majority of the material does not approach
"gem quality". Inherent: Other minerals occur in only a few locations or in very
small deposits. Inherently rare gems are doubly rare as the fraction
of an already small amount of ore which is gem quality is very, very,
A gem with relative rarity
A gem with inherent rarity
The mineral corundum (of which ruby is a gem example) is widespread
and abundant. So much so, that an enormous amount of low grade corundum
is used in industry for abrasives, due to its hardness (9 on a scale
of 10). [Interestingly, very tiny, non-gem grade, corundum crystals
have found use in today's beauty industry as the active ingredient in
both medical "dermabrasion"agents, and over the counter "exfoliating"
"Specimen" grade corundum $50 per pound (= @ 2 cents per
on the other hand, is found in gem quality in only one location on Earth:
the San Benito River Valley in California. Only a few ounces of cut
gems result from each year's mining efforts, almost all of which are
quite small in size. Ironically, this ultra-rare, nearly unobtainable
stone has been officially designated as the State Gemstone of California.
Pyramid of Gem rarity
in a deposit of gem mineral bearing ore, the majority is not the mineral
being sought. From the small portion of the ore which bears the gem
mineral, the majority is too low grade to have any gem uses. For example,
80% of the diamond recovered from diamond bearing ore, is industrial
the small amount of gem grade material, the bulk of it is of lowest
quality and useable only for inexpensive beads or trinkets. The even
smaller amount of better material which can be extracted, is mostly
middle grade, or that which is used for cabochons and better beads and
A tiny fraction is high grade and can be used for faceting. Most of
the facet grade material has some defects in color or clarity that limit
it to "commercial" quality gems. Only the most miniscule part of the
original deposit is top grade : AAA color and flawless clarity.
the starting amount at the base of the pyramid for a gem like ruby is
much larger than the starting amount for a gem like Benitoite, the amount
at the top is correspondingly larger.
Finally, picture taking that top "highest grade" part of the pyramid
and dividing it, again, into layers based on size: from small at the
base, to large at the tip. Is it any wonder that the largest, finest
gems bring astronomical prices?
Speaking of prices: How valuable are gemstones? If you ask people
at random to name a valuable commodity, many might say gold. And true,
we do think of gold as valuable.
Consider this :-
quality amethyst gems sell for about $40/ct
quality aquamarine sells for around $200/ct
gem quality blue sapphire sells for as much as $2500/ct.
gold, however, is worth well under $10 per carat! Down through the centuries,
gemstones have respresented the ultimate in portable wealth. (In the
next lesson, we'll go through the calculation that produced the cost
of gold figure). A
gem is durable. It must be strong enough to withstand the stresses and
forces involved in fashioning it, and its subsequent use as an ornamental
object, or in jewelry. Most everyone has heard of "hardness" and knows
that harder is better, in terms of using gems for jewelry but in reality,
hardness is only the beginning of the story. There are two other aspects
of gem durability that are at least as important as hardness.
Three Aspects of Durability :-
is the ability to resist scratching. Commonly measured on the
"Mohs" Scale of.
- 10 Talc
lowest (1), diamond highest (10). Soft gems, especially those
below 7 will tend to become dull through abrasion with harder
materials in the environment, and lose their surface polish and
their crisp edges over time.
is the ability to resist breaking or chipping. This property is
terms rather than on a numeric
scale: sphalerite is fragile, diamond is moderately tough and
jade is exceptionally tough. The lower the toughness of a gem
the more susceptible it is to damage by the kinds of blows and
knocks that are inevitable with frequent wear and use.
is resistance to changes caused by environmental factors such
and light. Apatite is temperature sensitive,
pearls are chemically sensitive, and Kunzite's color is unstable
in strong light. Unstable gems exposed to common factors of the
natural or man-made environment are likely to break, change color,
or lose their luster.
Classifying Gems :
are any number of ways by which gems can be classified. The remainder
of this presentation describes several of the most common ways :
OR SEMIPRECIOUS: (HISTORICAL VIEW OF VALUE)
OR CABOCHON: (CUTTING STYLE)
OR SYNTHETIC: (ORIGIN)
OR UNENHANCED: (TREATMENT STATUS)
OR FAKE: ( HOW REPRESENTED)
STONE OR DIAMOND: (GEM INDUSTRY VIEW)
OR COLLECTOR GEM: (WHO WILL BE THE END USER)
Classified By Historical View of Value: "Precious or Semiprecious".
These terms were routinely used (until about the 1980's) to separate
diamond, ruby, sapphire, emerald and sometimes pearl, from all other
gem species such as tourmaline, jasper and amber. Most gemologists no
longer use these words and consider them out-moded. Why?
The term, "precious", implies rarity and high value, but, in reality,
the individual specimens of each gem species and variety exist within
in a full spectrum of rarity, and of value, from very low to very high.
Some pieces of "semiprecious" gems are rarer, and more valuable than
some individual specimens of "precious" gems.
Ruby (in zoisite)
what should we call them then? Simple: gemstones or gems. These
terms will cover them all, regardless of where a given piece lies within
the continuum of rarity, beauty, and value for its species.
seriously is this idea taken that the "Code of Ethics and Principles
of Fair Business Practices" of the American Gem Trade Association (a
colored stone trade organization) instructs members to "avoid the use
of the term "semiprecious" in describing gemstones", and they have purged
that term from all their publications.
By Cutting Style: Faceted or cabochon cut: are the two most common ways
in which gems are fashioned.
Faceted stones are usually cut from transparent rough of relatively high clarity.
They are fashioned with a top (crown) and a bottom (pavilion) that have
intersecting flat planes called facets, on their surfaces. These facets
have shapes that are generally triangular, kite shaped or rectangular.
cutting is most often used for translucent
and opaque gems and such pieces generally have a flat bottom and a smoothly
curved top called a dome.
Cabochon cut lapis lazuli
parts of a faceted gem :-
Girdle : The girdle is the divider between the
top and bottom of the gem. It defines the face-up outline, and the maximum
dimensions of a faceted gem. In well proportioned stones, it usually
comprises about 2% of the total depth of the gem.
: The top, the part of the gem above the girdle is known as
its crown. In a well proportioned stone it makes up 1/4 to 1/3 of the
total depth of the gem. Table:
The largest, usually central, facet on the crown of a faceted gem is
the table. Generally, it makes up between 40 - 70% of the crown diameter.
Pavilion : The pavilion is the bottom, the part of the gem
below the girdle. In a well proportioned gem, it usually accounts for
2/3 to 3/4 of the total depth of the gem.
Pavilion view diagrams of round and emerald cut faceted
Culet/Keel : The tip or line at the bottom of the pavilion
on a faceted stone where the pavilion facets meet.
on a square cut stone
on an emerald cut stone
Classified By Origin :-
Natural : A natural gem is one produced entirely by geologic and/or biological
processes without any human input or assistance.
Synthetic : laboratory grown, manufactured, or "cultured" by human intervention.
A synthetic can be a copy of a natural mineral such as corundum, amethyst,
or pearl, or it can be a unique material not found in nature like YAG
(yttrium aluminum garnet) or cubic zirconia. In addition to the use
of synthetics as gem substitutes, they are also made for industrial,
electronic, and research purposes. Examples include synthetic diamonds
used as abrasives, and synthetic rubies and garnets used in lasers.
Classified By Treatment :-
Unenhanced : means (except for cleaning and/or fashioning into a useable gem)
that the material is as it was yielded from Nature. The color, transparency,
hardness, or optical phenomena have not been changed by man.
Among the general public "natural" is often misunderstood to mean
unenhanced but, in the reality of the gem trade, the term natural, does
NOT equal the term unenhanced. When gems are formally described and
graded, the origin (natural-vs-synthetic) is a totally separate factor
from the treatment status (unenhanced-vs-enhanced).
Unfortunately, many sellers know that by using the word "natural" in
the description of a gem (which may be true) their buyers will assume
that the gem is untreated (which is very likely not to be true).
Routinely enhanced gemstones : black onyx (dyed to change
color), emerald (oiled to increase clarity), sapphire (heated to change
color), blue topaz (irradiated, then heated to change color)].
Enhanced : an enhanced gem has received some type of treatment to change
its characteristics: Ex. irradiation, heating, dyeing, oiling, laser
drilling, etc. There are numerous treatments, some of which are routine,
have little effect on value, and are considered acceptable as long as
they are disclosed to the buyer, and others which are considered extreme
and which dramatically alter the value of the gem. A treatment may increase,
decrease or have no effect on the durability of a gem. (In a later web
lecture, gem enhancement will be covered in detail.)
The examples below are of some of the most common, well accepted treatments
that have minimal effect on gem value.
by intended use Simulant or Fake :-
Simulant : A material, either natural or synthetic, which is being used to
imitate another material. Simulants look like what they imitate, but
they may or may not share its chemical, physical and optical properties.
Not all simulants are synthetics! These mimics are correctly termed
either "simulant", "imitation", or "faux".
synthetic ruby can be used to simulate a natural ruby, but it
is also possible for natural red spinel to be used to simulate
a natural ruby.
Fake : Any material which is represented as something it is not.
The fake can be of man-made, or natural origin. Whether something is
a fake or not, is simply a matter of "truth in advertising".
Not Fakes : A ynthetic ruby offered as a
synthetic ruby. Man-made red glass offered as a "faux" ruby. A cubic
zirconia offered as a "diamond simulant".
A natural red garnet offered as a ruby.
man made Moissanite offered as a diamond.
enhanced colored diamond offered as an unenhanced colored diamond.
Simple test : If the material is represented accurately,
it is not a fake, if it is represented inaccurately it is a fake, regardless
of whether it is natural or man-made.
By Industry Terms : Colored Stones or Diamonds: Gemologists put all colored stones together into one category
and all diamonds into a separate one regardless of their color! The
reasons for this is that there are great differences which exist in
the systems for fashioning, grading and marketing these two categories
of gems. This distinction doesn't divide cleanly, however, between all
stones that show color, and all that are diamonds.
Some gems which are classified as "colored stones" are, in fact, colorless.
Examples would be white sapphire, white beryl, phenakite and rock crystal
quartz. Some diamonds have color, in fact, they are referred
to as "fancy" diamonds, amongst which we find the green, pink, blue,
yellow, orange, brown, red and black diamonds.
Classification is simple: is it a diamond?, No then it's a colored
stone (regardless of its color or lack of it!)
Rough rock Crystal Quartz
distinction is not as clear cut as some of the others. Although there
are over 3000 species of minerals, of which only 100 - 150 have the
characteristics that we associate with gems, and of these, only about
50 species make up a regular part of the jewelry marketplace. In reality,
though, the properties of jewelry and collector stones overlap and grade
into one another. In
general, a jewelry gem is one that is both durable enough to be used
for most jewelry applications, and common enough to be found in the
marketplace in at least moderate amounts. Aquamarine is a good example
of a jewelry gem. It is both durable and common to be
readily used, and is widely found in
the jewelry marketplace.
A collector gem is one that is either not durable enough to be
used in jewelry, or so rare that it is not found within the common market
channels for jewelry.
Transparent rhodocrostite is an example of a collector gem which,
is not durable enough to be set and worn in jewelry, although
it is abundant enough to have a place in the jewelry market if
it were useable. Clinohumite
is an example of a collector gem which is quite durable enough
for most jewelry uses, but so rare that only a few collectors are able
to obtain specimens, so it is not found within the normal gem
and jewelry channels.
Each of these items has some of the requisites for being a gem, but
lacks at least one of the crucial defining properties. A butterfly wing
is natural, beautiful, and may be rare, but it is not durable. Industrial
grade diamond is natural and durable, and is rare, but it is not beautiful
(which is why it is used only industrially), quartz beach sand is natural,
durable and if you were to examine it under a microscope you'd see it
is, in fact, beautiful, but it is not rare. A laboratory grown ruby
is beautiful and durable, but it is neither rare nor natural.