RUBIES
GEOLOGY.COM
What makes a Ruby?
Rubies are gem corundum with a dominant red color.
The color can range from orangy red to purplish red or
brownish red. The most desirable color range is a
pure vibrant red to a slightly purplish red (pigeons
blood).
The red color of ruby is produced by the presence of
chromium in the gem. A small trace of chromium will
produce a pink color. To be considered a ruby, there
must be enough chromium to give the gem a distinctly
red color.
Collection of rubies
Finally, rubies must have a combination of color and clarity that makes them an attractive gem. Opaque pieces of
corundum with just a hint of red color are not rubies – they are common corundum.
Mining Rubies
Most gem-grade corundum forms in metamorphic rocks such as schist or gneiss; or in igneous rocks such as basalt
or syenite. However, gem corundums are rarely mined from the rocks in which they form. Mining small gems
from hard rock is possible, but it is very expensive, and many of the gems are broken during the mining process.
Fortunately, corundum is very hard and resistant to weathering. In many areas, natural weathering and erosion
have liberated the stones from their host rock, and carried them into streams over long periods of geologic time.
Group of 3 rubies
Courtesy: B&B Fine Gems (Left & Right), Jeffrey Bergman (Centre)
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Today, the gems are mined from these
stream sediments. Their high specific
gravity relative to other sediment particles
often causes currents to concentrate them in
small placer deposits. Most rubies and
sapphires are produced by washing the
gravels of these stream deposits. This work
is often done by hand because the deposits
are small and irregular in shape and
character. These deposits are often located
in countries where wages are very low and
artisanal mining is prevalent.
Noteworthy locations where gem-quality corundums have been produced include Mayanmar, Thailand, Cambodia,
Vietnam, India, Pakistan, Afghanistan, Sri Lanka, China, Australia, Madagascar, Kenya, Tanzania, Nigeria, and
Malawi.
Ruby Treatments
Very few specimens of corundum have a natural color within the range required for a ruby. Very few also have
the clarity required to produce a nice faceted stone. Long ago, people who prepare gem materials for cutting began
experimenting with ways to improve their color and clarity.