What is a Gem Introductory ideas o o o o o 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 o o o o o 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 o 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 Why are gems cut the way they are Click for larger image 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 Internal reflection critical angle 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 Refraction 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 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 Fire 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 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