Jade is actually a group of two types of minerals - Nephrite and Jadeite.
Jadeite is a little harder than Nephrite although a little more brittle - and it was only around the late 18th Century that it was determined by a French minerologist that Jade was in fact 2 different minerals. Nephrite is normally green to creamy white - whereas Jadeite shows many colour variations including blue, lavender, pink and emerald-green and is jadeite is rarer. Translucent emerald-green jade is the most prized variety.
The hardness varies from 6.5 to 7 on the Mohs scale, and it is a pyroxene mineral with a composition formed of Sodium, Aluminium silicate in the case of Jadeite and Calcium Magnesium Iron Silicate in the form of Nephrite with a hardness of 6 to 6.5.
Jade may sometimes be enhanced (called Stabilized) in the following grades.
Type A Jadeite has not been treated except for surface waxing
Type B Jadeite may have chemical bleaches and clear polymer resin (significant improvement in transparency and colour)
Type C Jadeite may have been artificially stained - resulting in a loss of translucency
Type D Jade refers to a composite stone such as jade with plastic backing.
Here are some typical examples of Jade.