Emeralds are actually more rare than diamonds. They are a variety of the mineral Beryl. Pure beryl has no color and it is due to traces of chromium and vanadium that the green effect so prized in emeralds comes about. Emeralds occur in ranging hues but the gems that are medium to dark in tone (green) are considered emerald.
They have a hardness of 7.5-8 out of 10 on the Moh’s scale. Emeralds are highly included, so their toughness (resistance to breakage) is classified as generally poor so you should not put them in an ultra sonic and drastic change in temperature can break them. Emeralds tend to have numerous inclusions and surface breaking fissures.
A loupe is not used to grade an emerald, it is graded by eye. If an emerald has no visible inclusions (to the eye) it is considered flawless. Most emeralds are oiled after they are cut, in order to improve their clarity. Cedar oil, having a similar refractive index, is often used in this generally accepted practice.
This article, written by Valerie Heck Esmont, was featured in the Ohio Jewelry and Metal Arts Guild Newsletter.