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Emeralds have a hardness of 7.5 on the 10 point Mohs scale of hardness. However,
this Mohs rating can decrease, depending on the number and severity of inclusions
Most emeralds are oiled as part of the post lapidary process. The amount of oil
entering an emerald microfissure is roughly equivalent to the size of a period in
print.
Emeralds come in many shades of green and bluish green. There is a wide spectrum
of clarity, along with various numbers of inclusions. Most emeralds are highly
included, so it is quite rare to find an emerald with only minor inclusions. Because of
the usual inclusions, the toughness (resistance to breakage) is classified as
generally poor.
Emeralds in antiquity were mined by the Egyptians and in Austria as well as Swat in
northern Pakistan.
A rare type of emerald known as a trapiche emerald is occasionally found in the
mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has ray-like
spokes of dark carbon impurities that give the emerald a six-pointed radial pattern.
It is named for the trapiche, a grinding wheel used to process sugarcane in the region.
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