The Probert Encyclopaedia of Rocks & Minerals

JADE

Jade is a compact, opaque gemstone ranging in colour from dark green to almost white. The term is applied to specimens cut from the minerals jadeite and nephrite.
Jadeite, the less common and more highly prized of the two minerals, is a silicate of sodium and aluminium, NaAl(SiO3)2, usually containing some iron, calcium, and magnesium. It belongs to the group of minerals called pyroxenes.
Jadeite crystallizes in the monoclinic system but rarely occurs in distinct crystals and is usually found in fibrous, compact, massive aggregates. It has a hardness ranging from 6.5 to 7; it is extremely tough and difficult to break. The lustre on fresh fracture is dull and wax like, but polished jadeite has a vitreous lustre. Jadeite is found chiefly in eastern Asia in Burma, as well as in sections of Tibet and southern China. Nephrite, a member of the amphibole group of minerals, is a silicate of calcium and magnesium, with a small amount of iron replacing part of the magnesium. It is a tough, compact variety of the mineral tremolite with a hardness of 6 to 6.5. Polished nephrite has an oily lustre. It is found in Alaska, Mexico, New Zealand, Siberia, and Turkestan.
Jade was used in ancient times for weapons, utensils, and ornaments. A variety of jade called axstone is used by the natives of the South Sea islands for making hatchets. Jade has always been prized by the Chinese and Japanese as the most precious of all stones, and the most beautiful 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. Jade is a highly valued gemstone used in jewellery.
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JAMESONITE

Jamesonite, named after Professor Jameson, of Edinburgh, is a steel-grey mineral, of metallic lustre, commonly fibrous massive. It is a sulphide of antimony and lead, with a little iron.
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JARGON

Jargon (also known as Jargoon or Jacynth or Matara Diamond) is a colourless, yellowish or smoky coloured variety of zircon found in Sri Lanka.
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JAROSITE

Jarosite, named after Barranco Jaroso in Spain, is an ochre-yellow mineral occurring in minute rhombohedral crystals. It is a hydrous sulphate of iron and potash.
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JASPER

Jasper is an opaque, cryptocrystalline variety of quartz that takes a high polish and is used as a gemstone. It is usually stained by impurities and occurs in various colours, such as red, green, yellow, and blue. When the colours are arranged in bands, the mineral is called riband jasper; a variety containing alternating bands of red and green is known as Siberian
jasper. Mottled yellow or brown varieties of jasper are called Egyptian
jasper. Agate jasper is intermediate in structure between true jasper and chalcedony . Inclusions of red jasper occur in heliotrope. The jasper mentioned in the Bible as one of the stones in the breastplate of the high priest and as the foundation of the wall of the New Jerusalem is believed to have been a dark green, opalescent stone. The jasper of the ancients was a partially translucent stone, probably containing some chalcedony and a variety of the latter known as chrysoprase.
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JASPER OPAL

The jasper opal is a yellow variety of opal resembling jasper.
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JASPILITE

Jaspilite is a compact siliceous rock resembling jasper.
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JASPONYX

Jasponyx is an onyx, part or all of whose layers consist of jasper.
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JEFFERSONITE

Jeffersonite, named after Thomas Jefferson, is a variety of pyroxene of olive-green colour passing into brown. It contains zinc.
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JET

Jet is a variety of lignite, of a very compact texture and velvet black colour, susceptible of a good polish, and often wrought into mourning jewellery, toys, buttons, etc. It was formerly also called black amber.
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JURASSIC

The Jurassic was the second geological period of the Mesozoic era. It followed the Triassic, which ended about 190 million years ago, and extended until the beginning of the Cretaceous period, about 139 million years ago. It was named in 1829 by A Brongniart after the Jura Mountains on the borders of France and Switzerland.
Jurassic rocks include clays and limestones in which fossil flora and fauna are abundant. Plants included ferns, cycads, ginkgos, rushes, and conifers. Important invertebrates included ammonites (on which the
Jurassic is zoned), corals, brachiopods, bivalves, and echinoids. Reptiles dominated the vertebrates and the first flying reptiles - the pterosaurs - appeared. The first primitive bird, Archaeopteryx, also made its appearance during this period.
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