Jade

Jade, general term for the two silicate minerals jadeite and nephrite, which range in colour from dark green to almost white, and which are prized as semi-precious gem and ornamental stones. Both are compact, tough, fine-grained minerals and for centuries jade was thought to be a single gemstone; it was not until 1863 that the two varieties were recognized. The toughness of jadeite and nephrite is a result of the interlocking structure of their fibrous, monoclinic crystals, which makes pieces extremely difficult to break and thus particularly suitable for carving. In fact, jade was used in ancient times for weapons, utensils, and ornaments; the earliest jade carvings date from the New Stone Age in China (between c. 4000 and c. 2000 bc). Jade has always been prized by the Chinese as the most precious of all stones. The best specimens of carved jade, in the form of ornamental pieces such as vases, bowls, tablets, and statues (many of which are now museum pieces), were made in China.

Jadeite, the rarer and more highly prized of the two minerals, is a silicate of sodium and aluminium, with the formula NaAl(SiO₃)₂. It belongs to the group of single-chain silicates called pyroxenes and is colourless when pure. However, the frequent presence of traces of other minerals, such as iron, calcium, and magnesium, means that it can be found in a variety of colours, including white, pink, lilac, brown, and yellow, as well as green. The most prized variety, called by the Chinese “imperial jade”, is a rich emerald green, the result of traces of chromium. Jadeite rarely occurs in distinct crystals, being normally found in fibrous, compact, massive aggregates. It is translucent to opaque, and has a hardness ranging from 6.5 to 7 and a relative density (specific gravity) ranging from 3.3 to 3.5. The lustre on fresh fracture is dull and waxen, but polished jadeite has a vitreous lustre. Jadeite is found in metamorphic rocks, especially schists, and as alluvial pebbles or boulders. The most important deposits are in Myanmar (Burma), the main source of imperial jade. Other sources of jadeite include California in the United States, Japan, Tibet, and southern China. Historically, Guatemala was an important source of the jadeite used by the Olmec, Maya, and Aztec civilizations of pre-Columbian Central America and Mexico for ceremonial axes, knives, masks, and animal figures.

Nephrite is the compact, fine-grained variety of the amphibole minerals actinolite and tremolite. It is a double-chain silicate of calcium and magnesium, with a small amount of iron replacing part of the magnesium; its chemical formula is Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂. Nephrite is translucent to opaque, and has a hardness of 6 to 6.5 and a relative density of 2.96 to 3.1. Iron-rich varieties (actinolite) are dark green in colour, while at the other end of the range, magnesium-rich (tremolite) nephrite is cream or grey. Polished nephrite has an oily lustre. Nephrite is found as fibrous aggregates in metamorphic rocks, notably impure dolomitic limestones (tremolite) and low- to medium-grade schists (actinolite). The specific interlocking structure of its crystals makes it tougher than steel and nephrite has been used since prehistoric times to make weapons—indeed, it was formerly known as axestone. In New Zealand, the Maori traditionally used it to make decorated clubs called meres, as well as ornaments. Significant deposits occur in Alaska in the United States, Australia, Brazil, Canada, Germany, Mexico, Myanmar (Burma), New Zealand, Poland, Siberia, Turkistan, and Zimbabwe.