|Teapot made of "Mutton-Fat" Nephrite Jade|
Photo credit: Cha Dao
Nephrite jade, long coveted by Chinese Emperors, is a monoclinic amphibole which forms in a variety of colors, including green, yellow, white, grey, and brown-to-black. Amphiboles are generally composed of double chain silica and oxygen chains linked at vertices infused with iron and/or magnesium ions within the crystal structures. Generally, they form as dark-colored rocks. Though nephrite is not always dark in color, it is most definitely classified as an amphibole containing calcium, iron, and magnesium.
In its purest state, nephrite jade is white or yellowish-white, which the Chinese affectionately call "mutton-fat." This rare hue of nephrite, highly prized by the Chinese elite, is almost pure tremolite (a calcium and magnesium silicate in the amphibole family). Most nephrite contains trace amount of several other mineral substances, including diopside, garnet, magnetite, chromite, graphite, and others. The various compositions of these minerals within the stone called nephrite places them all in a series of amphiboles called Ferro-Actinolite-Tremolites.
As with its twin jadeite, nephrite jade owes its varied colors to trace amounts of chromium (emerald green) or graduating amounts of iron replacing some of the magnesium in the crystal structures. On the actinolite side (magnesium-rich amphibole) of the spectrum, nephrite will be green, yellow, or brown in color. Whereas, sliding over toward tremolite (calcium-rich amphibole), nephrite runs white or grey.
Nephrite is celebrated for its toughness. It has often been carved into axe heads, knife blades, and various tools for chopping. It has also long been the material of choice for Asian sculptors. It owes its durability to bundles of fine, fibrous crystals meshed together in felted layers. Under magnification, the tangled fibers woven together take on the appearance of rope.
Both tremolites and actinolites contain water molecules. One way to determine if a piece of green jade is nephrite or jadeite is to heat it in a closed glass container. If steam forms on the sides, it is likely that you have a piece of nephrite on your hands.
Read more about Jade Formation
Read more about Jadeite Jade
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*Clip art courtesy of The Graphics Fairy