What is a Gem?
In this course we will consider what gems are, the factors that affect
their value, where gems form, how gems are identified, why gems are colored,
and other important gemology concepts such as simulants, synthetics, gemstone
enhancement, and related issues.
Gems have been part of human history for over 20,000 years.
very early gems were generally of organic materials.
(left-right) coral, amber, and vegetable ivory (tagua nuts).
most gems used today are inorganic minerals.
early crystal gems were probably derived from alluvial sources.
as found, gems are rather ordinary-looking, unlike cut gemstones:
there are many different kinds of gems, and most come in many colors
gems can be synthesized
gems can be enhanced (and most commonly are)
simulants are different from synthetics
names: trade or commercial names obscure the true identity
a gemstone or simulant material
| A gem is a naturally
occuring material desirable for
its beauty, valuable
in its rarity, and sufficiently durable
to give lasting pleasure.
It should be naturally occuring, but
it need not be crystalline.
Beauty is determined by brilliance,
iridescence, color, sparkle, and play of color.
A gem should be durable against heat
and common household chemicals. It should not be easily scratched
or broken. Brittleness is a measure of the gem's tendency to crack
How rare is rare?:
Typically, a diamond deposit yields about 5g gem/1000kg of mined material.
That's 5g per million grams!
Beauty of a gemstone is determined by brilliance, luster, fire
and color (later lecture). The first three quantities depend on the
cut of the stone. Before we can understand why cut gems sparkle,
we need to learn some basic terms to describe cut stones.
Cut stone vocabulary:
Polished planar surfaces are referred to as facets.
The midline of a facetted gem is called the girdle and may or may not
The area above the girdle is called the crown; the factes on the
crown are the table, the star facets, the kite (or bezel) facets and
the upper girdle facets.
The area below the girdle is called the pavillion; these facets are
known as the lower girdle facets, the pavilion facets and the culet.
The type of cut where gems have a flat bottom surface and a rounded upper
surface is called cabochon.
Click for larger image.
Why are gems cut the way they are?
We use these facts to determine how facets should be placed in order to
the path of light in a gemstone!
Reflection and refraction
In order to understand why gems are faceted, it is essential to understand
how light behaves once it passes into a gemstone.
Light can either be reflected off a surface or pass through the
surface into the new substance.
When light passes from one material into another, it is bent or refracted.
But by how much?
The amount light is bent is determined by the density difference between
the gem and air. A measure of the amount light is bent is termed the
index" or 'RI'.
The Critical Angle
The critical angle is the angle at which total internal reflection
is achieved. But what do we mean by "internal
Light travelling through a stone intersects the stone-air surface. If
it passes within the critical angle (measured relative to the normal to
that surface), it will exit the stone. If it passes outside the critical
angle, it will be internally reflected.
Naturally, in order to achieve brilliance and sparkle, we do NOT want
light to escape from the pavillion. We DO want light to escape from the
Thus, to recap, the placement of facets on a gem is determined
using critical angle information, which comes from the refractive
Many gem cuts that meet the basic critical
angle requirements can be created.
Two important examples are the "Brilliant Cut"
and "Emerald Cut".
For this course, we are not concerned about how facets are created in
practice. However, take a look here
if you are curious!
Watch these MOVIES
to see how this works!
Not only does the placement of the facets matter, but the
smoothness of the surface (called "luster") does too.
Luster is a function of both the surface and the RI of the mineral
itself. Terms used to describe luster include adamantine, pearly,
metallic, silky, vitreous, resinous, and waxy. Gem grading reports refer
to "finish" or "polish" to describe how well polished the surface is.
"Luster" is also used to describe how mirror=like the surface of a pearl
- When the surface
of the gem is polished, the light is internally
reflected, as expected.
If the surface of the gem is left rough, light
is lost through unplanned leakage.
"Fire" refers to the rainbow-like flashes of color seen in cut stones.
Fire is especially obvious in diamonds.
Another example: the rainbows should
Where do these come from?
It is important to realize that the extent to which light is refracted
(bent on passing into or out of the gem) is dependent upon the wavelength
(color) of the light. Note that blue light is
bent more than red light
The phenomenon of different amount of bending of different colored light
is referred to as dispersion.
Dispersion is measured:
dispersion = refractive index of violet - refractive index of red light.
Dispersion varies greatly with the mineral type. Lists
dispersion values are available
The fire of a gem is a consequence of the cut
of the stone, coupled with its dispersion.
Many of the light behaviors we have thought about here (reflection,
refraction, dispersion) are commonly observed in everyday life! Excellent
examples can be found in the atmosphere.
Fire in diamonds
Some minerals (such as those formed by evaporation
of sea water) dissolve easily and clearly these would be poor gem materials.
Resistance to scratching: this is evaluated by consideration
of gem hardness. There are two measures of hardness: scratch
hardness and indentation hardness. Generally, we use the scratch hardness.
If we compare two different minerals, for example diamond and
quartz (the main ingredient in beach sand) we will find that quartz crystals
are readily scratched by diamond but diamonds can not be scratched by quartz.
Thus, diamond is much much harder than quartz.
Unfortunately, most minerals with hardness greater than 7 on Moh's hardness
scale are brittle. Hardness is not toughness -- even a diamond can be
Commonly available materials can be arranged into a sequence of increasing
hardness, e.g., talc-fingernail-copper coin-pocket knife-glass-steel file.
This can also be done with minerals. Moh arranged 10 minerals into a
sequence that is known as Moh's hardness scale. This scale has talc (found
in talcum powder) at the soft end and diamond at the hard end. The hardness
of talc is 1, quartz is 7, diamond is 10.
Minerals can break by irregular fracture (like bottle glass) or by cleaving.
The 4 factors that affect the value
of a gemstone are easily remembered as the "4 c's":
rarity of a gemstone is an important factor in determining
the value. However, some other things that affect value that are unrelated
to the 4c's and rarity. The supply of a specific type of gem can be controlled
to improve the value or a specific gem may greatly change in value due
to consumer demand or perceived investment potential. It is interesting
to look at the values of
specific gemstones and see
how these change over time.
Color: we will deal with the origin of color in gemstones in a separate
lecture. Clearly, color affects value. Some colors are more desirable than
others. In part, this is dictated by personal taste and in part by industry
standards (e.g., for diamonds).
Clarity: flaws (crack, inclusions) decrease the value of a
Cut: the ideal proportions for gems (to optimize brilliance and
fire) are not always to be found in a faceted stone. Poorly cut stones
have much lower value. Small errors in the placement of facets decrease
the value of a gem. For example,
Carat weight: bigger is not always better, but for otherwise equal
color, clarity, cut, the larger the stone will be more expensive!
1 carat = 0.2 g, thus 5 carat = 1 g <--- remember this!
Notice that the number of carats depends on density, so two different
types of gems of the same size will normally be a different number of carats!
The value of a gem may be much lower if its apparent clarity or color
has been improved by treatment. Furthermore, synthetic gems (made by humans)
have very much lower values than natural stones ... and beware! - the gemstone
is not always the material it is claimed to be: it may be a simulant (look-alike).
do you know?
In this course, we will discuss some of the things to look for. Many
people turn to a professional organization such at The Gemological Institute
of America (GIA) and American Gemological Laboratories (AGL) for the "final"
determination, especially for more expensive stones. These organizations
provide certificates that document the characteristics
of individual gems.
Next Lecture: Where
do gems form and where are they found?