Jade: Imperial green gem of the East (Part 3) – scientific properties of jade
Sonja McLahlan (UK)
In this third part of my series of articles on jade, I will cover the chemical and physical properties found in this gem that make it what it is and why it is so suitable for the wide variety of uses to which it is put. Jade is a polycrystalline gem and is a generic term for two different minerals: jadeite and nephrite. Both materials are very tough as they consist of closely packed and dense crystals. The main differences between the two types are in their chemical composition and the range of colours in which they are found. The toughness of jade is remarkable. It has a strength greater than steel and was put to work by many early civilizations for axes, knives and weapons.
The three images shown above are (C)opyright – Wm Mason: www.mysticmerchant.com.
Nephrite
The colour of nephrite comes in ranges from mid to dark green or grey-green. However, examples can also be found in tones of white, yellow and red. The colours in nephrite are affected by the amount of iron in it. Low levels of iron produce white, cream and grey colours, whereas green and brown shades are influenced by exposure to iron.
The chemical formula for nephrite is Ca2 (Mg, Fe) 5 Si8O22(OH)2, that is, a combination of calcium, magnesium, iron, silicon and oxygen. Nephrite is not actually classed as a mineral in itself, but is a variety of the mineral actinolite.
The chemical structure is amphibole silicate (magnesium/iron silicate) in the actinolite series. The name “actinolite” comes from the Greek word, “aktinos”, meaning “ray”, which makes reference to its often-fibrous nature. The nephrite variety is composed of fibrous crystals inter-twinned in a tough, compact mass and has an index of refraction of 1.62. Other actinolite varieties are quite different from nephrite.
Jadeite
This is much harder to find, and the stone exhibits varying shades that include green. However, it can also be found in white or pink, reds, blacks, browns and violets. The colour distributions in jadeite and nephrite can vary a great deal. The influence of the chemical chromium on jadeite produces the emerald green jade called “Imperial Jade”. The presence of the element manganese, along with jadeite, is thought to produce a range of violet colours. Red jadeite is caused by the presence of iron. Black jadeite comes from chromium and iron. Purple jadeite is caused by chromium, iron and cobalt.
Jadeite is a pyroxene or a crystalline silicate mineral containing two metallic oxides. The chemical formula of jadeite is NaAl(Si2O6), that is, sodium aluminium silicate. On Moh’s scale of hardness, jade rates at 6.5 to 7, which is about the same as quartz. With an index of refraction of 1.66, jadeite is almost never found as individual crystals and is composed of microscopic interlocking crystals that produce a very tough material.
The three images shown above are (C)opyright – Wm Mason: www.mysticmerchant.com.
In the next part of the series I will be looking at the spiritual, symbolic and medicinal uses of jade throughout the ages.
Classification of jade
When being applied to carvings and for use in jewellery, jade quality broken down into four clear characteristics:
- Tone and colour. The finest colours are penetrating and vivid. They should be pure and evenly distributed.
- Translucency and clarity. Jade ranges from nearly transparent to opaque. The highest quality has a consistent clarity like honey.
- Texture. The texture of jade ranges from fine to coarse. The best examples are clear and free of impurities.
- Cutting. Jadeite is cut in two forms: Cabochon for jewellery and carved designs. Cabochon quality is based on the above four qualities. Carved jade designs are also based on attractive aesthetics and good workmanship.