Ruby, Emerald and Sapphire - What, Where, How
The traditional classification in the West, which goes back to the Ancient
Greeks, begins with a distinction between precious and semi-precious
stones; similar distinctions are made in other cultures. The precious stones
are diamond, ruby, emerald andsapphire,
with all other gemstones being semi-precious. This
distinction is unscientific and reflects the rarity of the respective stones in
ancient times, as well as their quality - all are translucent with
fine color in their purest forms, except for the colorless diamond, and very
hardnesses of 8-10 on the Mohs
scale. Other stones are classified by their color, translucency and
hardness. The traditional distinction does not necessarily reflect modern
values, for example, while garnets are relatively inexpensive, a green garnet
called Tsavorite, can be far more valuable than an mid-quality emerald. Another
unscientific term for semi-precious gemstones used in art
history and archaeology is hardstone.
In modern times gemstones are identified by gemologists,
who describe gems and their characteristics using technical
terminology specific to the field of gemology.
The first characteristic a gemologist uses
to identify a gemstone is its chemical
composition. For example, diamonds are made of carbon (C)
and rubies of aluminium oxide
Next, many gems are crystals which are classified by their crystal
system such as cubic or trigonal or monoclinic.
Another term used is habit,
the form the gem is usually found in. For example diamonds, which have a cubic
crystal system, are often found as octahedrons.
Gemstones are classified into different groups, species,
For example, ruby is the red variety of the species corundum,
while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), bixbite (red), goshenite(colorless), heliodor (yellow),
and morganite (pink)
are all varieties of the mineral species beryl.
Gems are characterized in terms of refractive
index, dispersion, specific
gravity, hardness, cleavage, fracture,
They may exhibit pleochroism or double
refraction. They may have luminescence and
a distinctive absorption
Material or flaws within a stone may be present as inclusions.