In China a wide variety of translucent to opaque, green stones are generically referred to as "jade", however there are only two minerals - nephrite and jadeite - that are officially accepted as such and they can occur in many colours other than green. Both are hard and durable materials that have been used to carve both decorative and functional objects. Being different minerals, they have differing chemical compositions and structures, with nephrite jade generally being composed of interlocking fibrous crystals and jadeite jade generally being composed of interlocking, more granular crystals. However, they can sometimes be very similar in appearance which then require testing to separate. In their finest qualities, jadeite jade is the more highly valued.
The dull green varieties of both nephrite and jadeite owe most of the colour to the presence of iron impurities but the bright emerald green, translucent variety of jadeite is coloured by chromium and is known as "Imperial Jade". Imperial Jade is the most highly prized of all jade and is especially treasured by the Chinese.