Jade is a term used to describe two different types of gemstones: nephrite and jadeite. Nephrite is a silicate of calcium and magnesium, while jadeite is a silicate of sodium and aluminum. Both minerals are found in a variety of colors, including green, white, yellow, brown, and black.Jadeite is a sodium-rich aluminous pyroxene; nephrite is a fine-grained, calcium-rich, magnesium, iron, aluminous amphibole. All jade is composed of fine-grained, highly intergrown, interlocking ("matted" or "felted" texture, like asbestos or felt) crystals of one or both of these minerals.
Jade is one of the toughest gem minerals known because of the intergrown nature of the individual crystals. The more precious kind of jade, this is a microcrystalline interlocking growth of crystals (not a fibrous matrix as nephrite is.) It only occurs in metamorphic rocks.
|The 18-ton Polar Pride boulder was
discovered at the Polar mine in 2000, Canada. |
Photo: Jade West Group
Nephrite jade is very tough, the toughest known natural mineral, three times tougher than ceramics and even tougher than steel. This explains why it was used in neolithic times for knife blades, axe heads and later for ornamental carvings. While it is not as hard as some other minerals like diamond, nephrite is made from an interwoven meshwork of fine fibres or needle-like crystals so is not brittle and does not break easily. Coarser crystals and alignment of minerals decreases the toughness of the mineral. Jadeite is the second toughest mineral known, but is not as tough as nephrite as its interlocking crystals are less fibrous.
Nephrite is by far the more common form of jade. Nephrite ranges in color from mid to dark green or grey-green, but it can also be white, yellowish or reddish. Nephrite is slightly softer than jadeite.
|imperial jade gemstone|
The rarest and most valuable jadeite is called imperial jade, colored by traces of chromium. It has color and transparency rivaling fine emerald, though imperial jade is slightly more yellow in tone than emerald.
Crystal System: Monoclinic
Habit: Commonly massive. Fibrous crystals, densely matted together. Usually found as water-washed pebbles or boulders. Rare botryoidal habit known.
Hardness: Nephrite: 6 - 6.5 Jadeite: 6.5 - 7
Toughness: Extremely tough
Cleavage:2 directions; not evident in jade
Specific Gravity: Nephrite: 2.90 - 3.02 Jadeite: 3.3 - 3.5
|Jadeite (left) and Nephrite (right).|
Formation of Jade
Jadeite and nephrite are only found in metamorphic rocks, formed in current or ancient subduction zones. Jadeite is found in rocks that formed under higher pressure than nephrite. Most known deposits of jadeite occur along, or near faults in serpentinites. Examples of this type of deposit are found in Myanmar, New Zealand, Canada, Taiwan, Guyana, Surinam, southern Europe, Russia and China. In Australia, nephrite is mostly found as long lenses, within marble of Early to Middle Proterozoic age (2500 - 1000 million years ago).
Nephrite is common products of low-level regional metamorphism (green shale facies), and contact metamorphism of basic and ultrabasic rocks and dolomites. Nephrite consists of a microcrystalline interlocking fibrous matrix of the calcium, magnesium-iron rich amphibole mineral series tremolite (calcium-magnesium)-ferroactinolite (calcium-magnesium-iron). The middle member of this series with an intermediate composition is called actinolite (the silky fibrous mineral form is one form of asbestos). The higher the iron content, the greener the colour. Tremolite occurs in metamorphosed dolomitic limestones and Actinolite in metamorphic greenschists/glaucophane schists.
Nephrite jade: trade names
Black nephrite jade
Black nephrite jade a generic term referring to all black or almost black nephrite jade, which is usually caused by microscopic inclusions of dark minerals, mainly graphite.
Hetian Jade is one of the classic Chinese jades. It consists almost entirely of tremolite, but can contain up to 5% by weight of impurities such as diopside, epidote, clinozoisite, olivine, dolomite, quartz, magnetite, pyrite, apatite, limonite and carbonaceous matter.
Kunlun Jade is one of the classic Chinese jades, also known as "Qinghai Jade". It consists of actinolite and tremolite in varying proportions, and generally also contains minor wollastonite. The most precious specimens have green-white patterns.
Lantian Jade is a variety of tremolite (white jade), which varies in color from white to yellow, gray, pale blue and light green. The colors are due to the inclusions of iron oxides (yellow), graphite (gray) and chlorite, epidote or minerals of the serpentine group (pale blue).
Manasi Jade is a green jade which consists mainly of tremolite with only minor actinolite. The green color is caused by microscopic inclusions of green silicates, mainly chlorite and minerals from the serpentine group, and it also frequently contains diopsides and grossulars.
Meiling Jade is a typical tremolite jade, in a variety of colors from white to gray. The white varieties are the most precious.
Tremolite Jade is a generic term, referring to all varieties of nephrite which consist mainly of tremolite. While all white jades are tremolite jades, not all tremolite jades are white.
Xiuyan Jade is a green jade, which contains both tremolite and actinolite in varying proportions. Some varieties may also contain antigorite (Zhang, 2002). One of the classic Chinese jades.
Jade jadeite: trade names
Fei cui jade
Fei cui jade is the most popular variety of jade jadeite. Rarely also blue, lavender-purple, black, red or yellow, depending on the impurities.
Fake Jade is a Green ornamental stones commonly mistaken for jade
- Transvaal Jade: A compact variety similar to green grossular jade containing up to 25% zoisite. Often called "hydrogrossulaire", the water content may be due to the zoisite component rather than a real hydrogrenate.
- Queensland Jade: is a natural green to yellow chrysoprase grown in Queensland, Australia.
- California jade: is an idocrase (vesuvianite), extracted in the Butte County, near Chico, in California, it is a translucent yellow-green stone also called "californite", it is proposed as an alternative to jade.
- African jade: is the other name for buddstone, a variety of fuschite and green serpentine from South Africa.