Jade is a beautiful gem stone. With its smooth luxurious texture, glossy semi transparent appearance, top it with a pleasant white or green color, jade has been part of the Chinese culture since the Neolithic time. From the very early on, it was regarded as a stone with mystic power and the medium of choice for religious expression. The association with religion also brought on the representation and association with power and wealth. This deep affection and the belief in the mystic power of jade linger on turning into thinking that jade cannot deteriorate even after several thousands years of burial. In a sense it is true. Jade pieces survive several thousands of years, much better than many other materials of the same period of time. But the deterioration is there if one looks. It is this deterioration that nature left on the jade surface that gives clues to the authenticity of the period the jade piece was carved. To understand these changes, one must know some fundamentals of the nephrite jade.
The two different types of jade are nephrite and jadeite. Jadeite is not native to China and did not come into China from Myanmar until the Ming dynasty. All jade pieces before the Ming dynasty are nephrite. Both jadeite and nephrite are minerals. Jadeite belongs to the pyroxene group, and nephrite belongs to the amphibole group. Both are inosilicate minerals, meaning they are silicate groups containing silicon and oxygen. They form prism or needle like crystals and contain iron and magnesium in their structure. Nephrite is a combination of two amphibole minerals, tremolite and actinolite in various ratio. Tremolite and actinolite are both calcium magnesium iron silicate with traces of sodium and aluminum. They share the same chemical formula, Ca²(Mg,Fe)5Si8O22(OH)², and similar physical properties with specific gravity of 2.95 (+.15, -.05). Both actinolite and tremolite have a hardness of Mosh scale 5.5 – 6, and nephrite has a Mosh scale 6 – 6.5. The color of tremolite are white, grey, black, yellow, green, pink and purple. The pink and purple varieties are mainly found in Lawrence county New York and are not found in China. Tremolite is white when there is more magnesium in its composition, and green when there is more iron. Iron is what gives the jade yellow to green color. Actinolite is iron rich with color of various shade of green to black. In nephrite, when the ratio of the magnesium rich tremolite is higher than the iron rich actinolite, it is white. Both of the above two samples are nephrite roughs. On the left is iron rich and green. On the right is magnesium rich and white. Today any nephrite with 90% magnesium rich tremolite, and therefore has a greasy white mutton fat color, is called Hetian regardless where it comes from.
Minerals form crystals. The nephrite crystals are interlocking fibrous needles, bound together radiating out in a fan like fashion. The space between the bundles is the cleavage. which in nephrite is in two directions at about 60 and 120 degrees angles. The actinolite and tremolite specimens above show the crystal arrangement with the fibrous bundles radiating out. Because the crystals are bound tightly together interlacing, nephrite does not break easily. They are however easily cracked and separate at the cleavage. The interrelationship of the nephrite crystals are shown below in the scanning electron microscopy photos (Photos taken by Yi Bao https://www.gia.edu/gems-gemology/fall-2014-nephrite-jade-guangxi-province-china)
Rocks and minerals have a micro pore system, a system of minute openings on the surface between crystal and grain of the mineral. Pores extend down ward in pore throats, an interconnected passage way like channels that follow the cleavage of the mineral. Micro cracks also occur parallel or against the cleavage. The opening of these micro pores have a width of 1-2 μ. As weathering progress, the micro cracks widen to 5-10 μ. Water in the environment entering these micro pores and micro cracks, and reacting with the amphibole is the main aspect of chemical weathering. It is inside this system weathering changes take place. Below is a picture of the micro pores http://wiki.aapg.org/Pore_system_fundamentals . More details on the micro pore system can also be obtained from this link. http://wiki.aapg.org/Pore_systems
Gem minerals including nephrite form inside the crust of the Earth, with the only exceptions of peridot and diamond which form in the mantle. Under high temperature and pressure of the earth crust, nephrite is formed by metasomatism of dolomite by intrusive magmatic fluids, or silicic rocks by serpentinite fluids. This results in nephrite being encased in igneous and or metamorphic rocks, mixed with small amount of other minerals, diopside, magnetite, chromite, graphite, apatite, rutile, pyrite, datolite, vesuvianite, prehnite, garnet, talc and sphene. In China, nephrite from the Kunlun mountains are frequently associate with Pyroxene rocks, and those from the Karakash valley in Xinjiang province are frequently with Hornblende rocks. Thus nephrite are encased in other rocks making them difficult to identify. Nephrite in the primary deposit are deep underground, encased in rocks and therefore in general are not mined due to the expense of necessity to remove millions tons of rocks to obtain the mineral. With volcanic and tectonic activities, part of the primary deposit breaks off and uplifted to the surface from the crust to become the secondary deposit. There are two types of secondary deposit, eluvial and alluvial. As the deposit is pushed to the surface, weathering breaks down the hard encasing rocks and becomes a mount of debris with the mineral inside, forming the eluvial deposit. Nephrite mostly come from such mines probably exclusively before the Qing Dynasty. Within these mines nephrite are found encased within other minerals and rocks (坑玉). Identifying rocks that contain nephrite within can post challenges to nephrite miners and hunters.
Alluvial is when the primary or secondary deposit further erodes, broken off and finds its way tumbling down a river or stream. Way to obtain the mineral is usually by panning, as in panning for gold, or dredging in a larger scale mining. Minerals can also be found in dry up river beds. In such case hydraulic washing is the way to obtain them. Nephrite pebbles in rivers are found among similar size pebbles and rocks, making them unsuitable for panning. To mine such river pebbles traditionally in China, several people walk abreast, wading across the shallow river. As they wade across, they feels the pebbles on the river bed with their feet. Experience tells them which pebble to pick up for examination. However some believe that it is more accurate to say when wadding across they look down to pick up what looks like a nephrite pebble. River nephrite pebbles are less angular, smooth, polish and smaller due to constant rolling and tumbling in the stream. Most of the pebbles have areas of reddish and other discoloration called skin. The recent high demand of nephrite jade in China has basically exhausted the Xinjiang deposits. With unabated demands price of nephrite pebbles sky rocketed. Escalating high price brings out fake nephrite pebbles. How to identify the fake from the real nephrite pebbles becomes a problem. China in 2015 puts out a paper title “Identification Characteristics of Weathering Crust for Nephrite Gravel and its Imitation. Hui Li , Xuan Wang, Yong Zhu, Zhengyu Zhou. 5th International Conference on Information Engineering for Mechanics and Materials (ICIMM 2015)”. In the paper the authors identify three distinct points of difference between the nephrite pebbles natural weathering from the artificial applied changes.
- Something resembling sweat pores are found on the surface of the natural weathered pebbles. These sweat pores are irregular, varied in shape and sizes, with some of these sweat pores connected, and some have smaller sweat pores within a larger one with the appearance of an island. These sweat pores are thought to come from collision with other pebbles and rocks in the stream. The artificial sweat pores are made from sand blasting, and the pores are uniform in size and shape with edges round and smooth, unlike the natural sweat pores.
- Nail pattern formation on the natural weathered pebbles. These pattern are thought to be from the nephrite breaking from he primary deposit and collision in the stream. The artificial ones are more smooth and uniform.
- Artificial color changes on the crust of the false nephrite pebbles are done by applying chemical on the surface, and hence they are brighter in color. Using ultraviolet-visible absorption spectroscopy, the absorption peak of the color changes on the natural nephrite pebbles show iron oxides F²-, F³-, whereas the applied color changes on the false pebbles do not contain any iron.
The sweat pores described by the authors are also seen on the buried nephrite jades. From the description, these should be etch pits result from natural chemical weathering, and not from the collision with other pebbles as stated. Nephrite is probably the hardest pebble in the stream. If they collide with other stones, they are more likely to crack than to chip. Weathering produces etch pits on nephrite surface with characteristics similar to what the paper describe. It is safe to say the pits found on the pebble are etch pits from weathering. It is hard to know what the authors refer to as the nail pattern. From the picture in the article, it seems to refer to the dendritic changes under the nephrite surface. The iron oxide formation is also seen in buried antique nephrite. Like all minerals and rocks, the nephrite pebbles also go through chemical weathering in nature. The three observations to identify the natural nephrite pebbles from the false ones are similar to the observations used to identified the burial nephrite jade from the fake ones. These surface changes are the result of natural chemical weathering of amphibole which we are going to explore next.