What is a Gem? 

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Introductory ideas:
    • Gems have been part of human history for over 20,000 years. 
    • very early gems were generally of organic materials.


      Examples include (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:


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    • 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 of a gemstone or simulant material
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.
 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 or cleave.
  • How rare is rare?:
    • Typically, a diamond deposit yields about 5g gem/1000kg of mined material.  That's 5g per million grams!

Beauty: 

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 be facetted.
  • 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?

  • 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 "refractive 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 reflection"?

      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.

We use these facts to determine how facets should be placed in order to control the path of light in a gemstone!

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 top facets!

Thus, to recap, the placement of facets on a gem is determined using critical angle information, which comes from the refractive index information.

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!
Internal reflection, critical angle.

Refraction

Watch these MOVIES to see how this works!

Luster

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 is.
  • 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

"Fire" refers to the rainbow-like flashes of color seen in cut stones.

Fire is especially obvious in diamonds. 
Another example: the rainbows should be obvious!

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 of 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


Fire in diamonds

Durability

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.

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.
 

Unfortunately, most minerals with hardness greater than 7 on Moh's hardness scale are brittle. Hardness is not toughness -- even a diamond can be broken.

Minerals can break by irregular fracture (like bottle glass) or by cleaving.

Value

The 4 factors that affect the value of a gemstone are easily remembered as the "4 c's":
  • 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 gemstone.
  • 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!

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    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!

Obviously, the 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.

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). How 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.






Syllabus

Next Lecture: Where do gems form and where are they found? 

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