Tuesday, December 28, 2010

# Specific Gravity and Refractive Index

As a buyer of gemstones, you will eventually hear the terms ‘specific gravity’ and ‘refractive index’.  Don’t feel overwhelmed!  These terms are much more simple than they sound.  They are just tools to help distinguish between different types of stones.  To explain, we will use Walter Schumann’s Gemstones of the World to help us.  First up: specific gravity.  Specific gravity is the weight of a particular material compared to the weight of the same amount of water.  Therefore, a gem with a specific gravity of 2.6 (quartz) will weigh 2.6 times as much as the equivalent amount of water.  Specific gravity of gemstones always lies between 1 and 7 (for a few references - diamonds are generally 3.47-3.55; rubies are between 3.97 and 4.08; and emeralds are between 2.67 and 2.78).  Let’s say you find an unknown red gem.  Upon performing a specific gravity test, you find the specific gravity to be 4.01.  A quick glance at a reference chart will tell you that you may indeed have a ruby!

“Refractive Index” is another way distinguish between different gems.  The technical definition, by Schumann: “[refraction] occurs when a ray of light leaves one medium (e.g. air) and enters obliquely into another medium (e.g. a gem) at the interface between the two mediums.”  Think about a glass of water with a straw in it.  When you look at it, the straw appears to be “bent” where the water and air meet.  This is a perfect example of light refraction.  “Refractive Index” is measured using the speed of light in air divided by the speed of light in the medium.  Different mediums have different speeds of light.  In air, for example, light travels at about 300,000 km/s.  Diamonds, however, slow light down to about 125,000 km/s.  Therefore, the refractive index of a diamond would be 300,000/125,000 = 2.4.  Each medium has it’s own refractive index.  Therefore, with the aid of a tool called a Refractometer, it is easy to determine the type of gemstone simply by its refractive index.  Sapphires, for example, have a RI of about 1.77.  Amethysts have an RI of about 1.55.  By learning the refractive index of an unidentified gem, you can get an idea what type of stone it may be.