W.A. BlackNephrite
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Gemological Testing
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Nephrite (chen yu or true jade), the traditional
Chinese jade, is mineralogically a monoclinic amphibole of variable
chemistry that ranges between the end members tremolite [Ca2Mg5Si8O22(OH)2] and ferroactinolite [Ca2Fe5Si8O22(OH)2]. The magnesium in tremolite is
often partially replaced by iron and the two minerals grade optically and
chemically one into the other.
The colour of nephrite varies from white, yellow,
green and brown to black, depending on the amount of iron substituting for
magnesium. Iron-poor tremolite is white, grey or green; whereas iron-rich
varieties (actinolite) are darker green, grey-green, and grading though to
black. Cowell jade (South Australia) is nephrite jade that has an iron
content of up to 7.9 per cent Fe203.
Although nephrite jade exhibits minor variations in
its mineralogy, its important property is its toughness. For this reason
it was used for axe heads and knife blades as well as for delicate,
durable carvings. Toughness in this context is the opposite of
brittleness, and is a quite different property to hardness, which is
simply resistance of a material to scratching. Diamond is the hardest
mineral, whereas nephrite is the toughest, rivalling artificial zirconium
ceramics. The toughness of nephrite is due to its interlocking meshwork of
fine fibres or needle-like crystals which are commonly 0.1-5.0 microns in
diameter and 20-150 microns long (one micron is one thousandth of a
millimetre). These fibres usually are arranged in bundles.
The toughest nephrite has random orientation of its
fibre bundles combined with a very fine grain size. The presence of
foliation (aligned fibres or fibre bundles), as well as a coarse grain
size reduces the toughness of nephrite.
Source
- J.Townsend
The
Australian Gemoligist
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