Figures 1-3. Views of the sample showing its waxy to subresinous lustre, dark green-grey external faces, and interesting shape, reminiscent of a segment of tree trunk. Note also the customized, hand-chiselled, red-stained wooden base. This sample, with its customized, carved, stained wood base, is representative of the Chinese love of stylized rocks of interesting shape. A few key points:

• Size: 18 x 12 cm, 15 cm high.
• Mass: circa 19 lbs 3 oz (8.72 kg).
• Estimated volume and density (both approximate): 2,860 cm³ / 3.05 g/cm³.
• Magnetism: minimal, magnetic susceptibility circa 0.3x10^-3 SI units.

"Rock of the Month #207, posted for September 2018" ---

Nephrite Jade: A Chinese Landscape Rock:

Rock type

A dense, pock-marked, dark green stone, visually reminiscent of a piece of tree trunk. The rock is very fine-grained, suggesting the metamorphosed equivalent of a mafic volcanic rock such as basalt. The estimated density of 3.05 g/cm³ is quite high, about 15% higher than quartz and quartz- and feldspar- rich rocks such as sandstone and granite, but lower than the much sought-after jadeite (pyroxene, known principally from Burma and Guatemala) form of jade (3.30-3.38 g/cm³). The large sample was weighed on precise bathroom scales, and its volume determined by water displacement in a large bucket. This approximate density estimate fits quite well with the range of nephrite (2.90-3.03 g/cm³). Nephrite is a rock composed of amphibole-family silicate minerals, the relevant members forming a continuum from magnesium-rich tremolite (2.99-3.03 g/cm³) and iron-rich actinolite (3.03-3.24 g/cm³). Tremolite-rich jade can be white "snow jade", a desirable variant. Higher actinolite content results in a typical dark green hue. The "feel" of the rock, its heft, smooth surface and general appearance, suggest that it may well consist largely of nephrite (amphibole) jade, a dark green variety found in British Columbia and elsewhere.

China and decorative stones

The Chinese aesthetic sense for rocks is widely divergent from the Western collector's interest in crystal form and colour. Rather, Chinese taste in decorative stones often emphasizes shape and form, texture and sheer size. Larger examples may be carved with Chinese characters which are commonly painted for visual effect. Chinese decorative stones are diverse (Chinese Red, 2012), from granites and volcanic rocks to limestone, and including the more exotic types suitable for very intricate carvings, such as jades and the mixture of clay minerals known as Qingtian stone. In China, the use of jade dates back thousands of years (Ming, 2009; Chinese Red, 2011; Waldersee et al., 2014), including the Prehistoric (Neolithic) period, as well as the early Xia, Shang and Western Zhou dynasties. According to Yu Ming (2009), jade is said to have been revered in China for 8,000 years. The admirable attributes of jade (ibid., pp.4-7) include silky lustre, uniformity, sonorous qualities, tough knitted texture and strength (in folklore, considered analogous to benevolence, justice, wisdom, courage and purity, respectively).

Jade geology

While many kinds of rock may be termed jade, only the jadeite and nephrite forms are true jade. Both are hard, very tough and quite dense. Jade is found, generally in small amounts, and more often as resistant boulders than in outcrop. Boulders estimated to weigh anything from a tonne to several hundred tonnes have been discovered (e.g., Flather, 1992). Jade is also mined from veins found in hosts such as ultramafic rocks (e.g., serpentinite, magnesite) and in metasomatically altered dolomitic marbles.

The Cowell deposits of South Australia are said to be the world's largest known deposit of high-quality nephrite jade, containing almost 90% of world reserves in a host rock of middle Proterozoic dolomitic marble (Flint and Dubowski, 1990). Formed in retrograde alteration of folded marble, Cowell nephrite contains only 30-40 ppm Cr (nephrite from other sites averages 1000 ppm), reflecting the marble source (as opposed to Cr-rich ultramafic igneous rocks). The Chuncheon nephrite deposit in Korea, another major deposit, has similar Precambrian host rocks to Cowell: dolomitic marble and amphibolite schist, intruded by the late Triassic, post-tectonic Chuncheon granite. Isotopic studies at Chuncheon suggest alteration (forming jade) by a fluid of largely meteoric (percolating surface water) origin. This is distinct from nephrite deposits associated with serpentinites, such as Fengtien in Taiwan, where O isotopes were mainly buffered by the host serpentinite (Yui and Kwon, 2002).

Distribution

Jade is widespread across the world, in such regions as: western North America, e.g., British Columbia, Yukon, Alaska, Wyoming and California, and in small amounts in Ontario, Newfoundland and Labrador; central America, e.g., Guatemala and Costa Rica; Burma; Taiwan and mainland China (e.g., Xinjiang, Gansu, Henan and Liaoning provinces); Korea and Japan; New Zealand and Australia; Iran and Turkey; and Europe (e.g., Austria and Poland).

Acknowledgements: The sample was acquired by its current owner in southeastern Ontario, and unfortunately there is no provenance. One would imagine that this piece must have an interesting Chinese-Canadian story behind it. Thanks to Jim Connor for making it available.

References

Chinese Red (2011) Chinese Jades. Chinese Red, Beijing, 2011.8, 187pp. (in Engl. and in Ch.).

Chinese Red (2012) China Stones. Chinese Red, Beijing, 2012.2, 163pp. (in Engl. and in Ch.).

Flather,P (1992) Yukon jade find may be of world record size. Northern Miner 78 no.21, 1-2, 27 July.

Flint,DJ and Dubowski,EA (1990) Cowell nephrite jade deposits. In `Geology of the Mineral Deposits of Australia and Papua New Guinea' (Hughes,FE editor), Australasian Institute of Mining and Metallurgy Monograph 14, 1828pp., 1059-1062.

Ming,Y (2009) Chinese Jade: Sacred, Imperial and Civil Forms. China Intercontinental Press, Haidian, Beijing, 145pp.

Waldersee,V, Robinson,PTYW and Zhu,T (2014) Chinese Jade Artifacts. China Pictorial Publishing House, Haidian, Beijing, 150pp.

Yui,T-F and Kwon,S-T (2002) Origin of a dolomite-related jade deposit at Chuncheon, Korea. Econ.Geol. 97, 593-601.