Jet | |
---|---|
General | |
Category | Mineraloid |
Formula (repeating unit) | Variable, but rich in carbon |
Identification | |
Color | Black, occasionally brown |
Cleavage | None |
Fracture | Conchoidal |
Tenacity | Brittle |
Mohs scale hardness | 2.5–4.0 |
Streak | Brown |
Specific gravity | 1.3–1.4 |
Optical properties | Isotropic |
Refractive index | 1.640–1.680 |
Dispersion | None; opaque |
Ultraviolet fluorescence | None |
Common impurities | Iron, sulfur |
References | [ citation needed ] |
Jet is a type of lignite, [1] the lowest rank of coal, and is a gemstone. Unlike many gemstones, jet is not a mineral, but is rather a mineraloid. [2] It is derived from wood that has changed under extreme pressure.
The English noun jet derives from the French word for the same material, jaiet (modern French jais ), ultimately referring to the ancient town of Gagae. [3] Jet is either black or dark brown, but may contain pyrite inclusions [4] which are of brassy colour and metallic lustre. The adjective "jet-black", meaning as dark a black as possible, derives from this material.
Jet is a product of decomposition of wood from millions of years ago, commonly the wood of trees of the family Araucariaceae. [5] Jet is found in two forms, hard and soft. [5] Hard jet is the result of carbon compression and salt water; soft jet may be the result of carbon compression and fresh water. [5] Despite the name they both occupy the same area of the Mohs scale with the difference being that soft jet is more likely to crack when exposed to changes in temperature. [6]
Jet is around 75% carbon and 12% oxygen with sulfur and hydrogen making up most of the balance. [7] Other elements are found at trace level and the exact ratios varying with the source; for example, Spanish jet contains more sulfur than Whitby jet. [7] Jet has a Mohs hardness ranging between 2.5 and 4 and a specific gravity of 1.30 to 1.34. The refractive index of jet is approximately 1.66. The touch of a red-hot needle should cause jet to emit an odour similar to coal. [8]
Jet may induce an electric charge like that of amber when rubbed. [7]
Jet is very easily cut using carving tools, but small pieces tend to break off, making it difficult to create fine details. It therefore takes an experienced lapidarist to execute more elaborate carvings.
The jet found at Whitby, in England, is the "Jet Rock" unit of the Mulgrave Shale Member, which is part of the Whitby Mudstone Formation. This jet deposit was formed approximately 181 million years ago, during the Toarcian age of the Early Jurassic epoch. [9] [10] [11] Whitby Jet is the fossilized wood from species similar to the extant Chilean pine ( Araucaria araucana ). [12] The deposit extends throughout North York Moors National Park. [13]
Jet has also been found in Kimmeridge shale seams in Dorset. [14]
Jet was mined from a number of areas of France including Montjardin and Roquevaire. [15] Raw jet was also imported from Spain. [15] In the 18th century there was a jet working industry based around Sainte-Colombe-sur-l'Hers and La Bastide-sur-l'Hers but this declined with the start of the 19th. [15] An 1871 plan to import raw French jet into Whitby was unsuccessful due to its poor quality. [15]
The jet found in Asturias, the biggest deposit in northern Spain, [16] is of Late Jurassic (Kimmeridgian) age, about 155 million years old. Asturian jet is a perhydrous coal that suffered an anomalous coalification process and presents great material stability over long periods of time. [17]
Jet is also found near Santiago de Compostela in northwest Spain [18]
The Navajo and Pueblo tribes of New Mexico were using regionally mined jet for jewellery and the ornamentation of weapons when early Spanish explorers reached the area in the 1500s. [19] Today these jet deposits are known as Acoma jet, for the Acoma Pueblo. Enormous coal deposits characterize the San Juan Basin of New Mexico and this geology is closely related to jet deposits mined in the Henry Mountains of Utah [20] and the Front Range of El Paso County, Colorado. [21]
Jet is also commercialized in Poland [22] and near Erzurum in Turkey, where it is known as oltu stone and is used to make prayer beads. [23]
The earliest known worked jet object is a 10,000 BC model of a botfly larva, from Baden-Württemberg, Germany, found among the Venuses of Petersfels. [24]
Jet has been used in Britain since the Neolithic period [25] It continued in use in Britain through the Bronze Age where it was used for necklace beads. [25] Jet necklaces following the plate and spacer design may have been based on Gold lunula. [26] During the Iron Age jet went out of fashion until the early 3rd century AD in Roman Britain. The end of Roman Britain marked the end of jet's ancient popularity. [25]
Early archaeologists (particularly Victorian) often failed to distinguish between jet and other jet-like materials [27] In particular in southern Britain the material described as jet was often Kimmeridge Shale. [27] [28] and some artifacts use more than one jet-like material. [29] For example the Pen y Bonc necklace combines two or three jet pieces with other dark material. [29]
Whitby jet was a popular material for jewellery in Roman Britain from the 3rd century onward. There is no evidence for Roman jet working in Whitby itself, [25] rather it was transferred to Eboracum (modern York) where considerable evidence for jet production has been found. [30] The collection of jet at this time was based on beachcombing rather than quarrying. [25] It was used in rings, hair pins, beads, bracelets, bangles, necklaces, and pendants, [25] many of which can be seen in the Yorkshire Museum. Jet rings tended to follow the styles of existing metal rings although there were exceptions. [31] Jet pendants were carved cameo style with Medusa head being a popular theme. [32]
Stylistic similarities with jet items found in the Rhineland, and lack of any evidence for local manufacture, suggest that Eboracum-produced items were exported to that area. [33] One item that has been found around the Rhine but not in Britain are jet bracelets that feature grooves with gold inserts. [34]
The Roman period saw its use as a magical material, frequently used in amulets and pendants because of its supposed protective qualities and ability to deflect the gaze of the evil eye. [35] Pliny the Elder suggests that "the kindling of jet drives off snakes and relieves suffocation of the uterus. Its fumes detect attempts to simulate a disabling illness or a state of virginity." [36] It has been referenced by other ancient writers including Solinus [37] and Galen.
Vikings made some use of jet including rings and miniature sculptures of animals with snakes being a prominent theme. [38] The source of the jet has not been confirmed although Whitby is the most likely possibility. [38]
Medieval jet use appears to have been largely limited to religious items such as crosses and Rosary beads. [39] During the period there was a belief that water drunk from jet bowls could help with labour. [40] A jet bowl held in the Museum of London may have been designed to allow for this. [41]
Jet became a valued costume accessory in the 16th century. Mary, Queen of Scots own jet buttons and clothes embroidered with jet beads. [42] Elizabeth I bought 1000 "black jet bugle drops" to embroider headdresses in 1587. [43] Anne of Denmark ordered a gown of "double burret" silk in June 1597 loaded with jet passementerie and 360 jet buttons. The gown was too heavy to wear and she ordered it to be remade with less jet. [44]
Jet as a gemstone became fashionable during the reign of Queen Victoria. [45] It originally became fashionable in the 1850s after the queen wore a necklace of it as part of mourning dress for Princess Victoria of Saxe-Coburg and Gotha. [45] Later the Queen wore Whitby jet as part of her mourning dress while mourning the death of Prince Albert. [12] [46] [47]
In some jewellery designs of the period jet was combined with cut steel. [48]
Jet use was at its highest in the early 1870s and from there it declined. [49] From above 1000 workers in the trade Whitby was down to 300 by 1884. [49] While jet substitutes may have had an impact this appears to have been in a large part due to changes in fashion with Art Nouveau making little use of black jewellery. [49] As the numbers fell the remaining manufactures tended to stick with existing styles rather than attempting to adapt to new fashions resulting in demand falling further. [49] Making tourist trinkets kept a few jewellers in work, but by the end of World War II only three remained, and the industry died out completely with their deaths. [49]
In Whitby the Victorian tradition continued up until the aftermath of World War II. [49] Jet jewellery (both vintage and new) was then to remain out of fashion until the late '70s. [50] In the '80s there was a fashion for jet beads and antique jet jewellery started to rise in value. [51] New jewellers took up the production of jet jewellery. [51]
Glass was used as a jet substitute during the peak of jet's popularity. [52] [53] When it was used in this way it was known as French jet or Vauxhall glass. [52] [53] Ebonite was also used as a jet substitute and initially looks very similar to jet, but it fades over time. [54] In some cases jet offcuts were mixed with glue and molded into jewelry. [54]
Anthracite (hard coal) is superficially similar to fine jet, and has been used to imitate it. This imitation is not always easy to distinguish from real jet.
Some museums have produced reproductions of jet artefacts in epoxy resin. [55]
Unlike black glass, which is cool to the touch, jet is not cool, due to its lower thermal conductivity. When rubbed against unglazed porcelain, true jet will leave a brown streak, although bog oak, vulcanite, and lignite will do the same. [56]
When non destructive testing is required X-ray fluorescence spectroscopy combined with visual inspection (including under high magnification) and X-ray imaging is generally effective although it can be difficult to differentiate jet from lignite [29]
Real jet, when placed in a flame, burns like coal and gives off a coal-like smell and produces soot. No other black "gemstone" behaves like this.
Amethyst is a violet variety of quartz. The name comes from the Koine Greek αμέθυστος amethystos from α-a-, "not" and μεθύσκωmethysko / μεθώmetho, "intoxicate", a reference to the belief that the stone protected its owner from drunkenness. Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent intoxication.
A bead is a small, decorative object that is formed in a variety of shapes and sizes of a material such as stone, bone, shell, glass, plastic, wood, or pearl and with a small hole for threading or stringing. Beads range in size from under 1 millimeter (0.039 in) to over 1 centimeter (0.39 in) in diameter.
Jewellery consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a western perspective, the term is restricted to durable ornaments, excluding flowers for example. For many centuries metal such as gold often combined with gemstones, has been the normal material for jewellery, but other materials such as glass, shells and other plant materials may be used.
A pearl is a hard, glistening object produced within the soft tissue of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carbonate in minute crystalline form, which has deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur. The finest quality of natural pearls have been highly valued as gemstones and objects of beauty for many centuries. Because of this, pearl has become a metaphor for something rare, fine, admirable and valuable.
Lignite, often referred to as brown coal, is a soft, brown, combustible sedimentary rock formed from naturally compressed peat. It has a carbon content around 25–35%, and is considered the lowest rank of coal due to its relatively low heat content. When removed from the ground, it contains a very high amount of moisture which partially explains its low carbon content. Lignite is mined all around the world and is used almost exclusively as a fuel for steam-electric power generation.
Carnelian is a brownish-red mineral commonly used as a semiprecious stone. Similar to carnelian is sard, which is generally harder and darker; the difference is not rigidly defined, and the two names are often used interchangeably. Both carnelian and sard are varieties of the silica mineral chalcedony colored by impurities of iron oxide. The color can vary greatly, ranging from pale orange to an intense almost-black coloration. Significant localities include Yanacodo (Peru); Ratnapura ; and Thailand. It has been found in Indonesia, Brazil, India, Russia (Siberia), and Germany. In the United States, the official State Gem of Maryland is also a variety of carnelian called Patuxent River stone.
Lapis lazuli, or lapis for short, is a deep-blue metamorphic rock used as a semi-precious stone that has been prized since antiquity for its intense color. Originating from the Persian word for the gem, lāžward, lapis lazuli is a rock composed primarily of the minerals lazurite, pyrite and calcite. As early as the 7th millennium BC, lapis lazuli was mined in the Sar-i Sang mines, in Shortugai, and in other mines in Badakhshan province in modern northeast Afghanistan. Lapis lazuli artifacts, dated to 7570 BC, have been found at Bhirrana, which is the oldest site of Indus Valley civilisation. Lapis was highly valued by the Indus Valley Civilisation. Lapis beads have been found at Neolithic burials in Mehrgarh, the Caucasus, and as far away as Mauritania. It was used in the funeral mask of Tutankhamun.
Opal is a hydrated amorphous form of silica (SiO2·nH2O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur in the fissures of almost any kind of rock, being most commonly found with limonite, sandstone, rhyolite, marl, and basalt.
Turquoise is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl6(PO4)4(OH)8·4H2O. It is rare and valuable in finer grades and has been prized as a gemstone for millennia due to its hue.
A necklace is an article of jewellery that is worn around the neck. Necklaces may have been one of the earliest types of adornment worn by humans. They often serve ceremonial, religious, magical, or funerary purposes and are also used as symbols of wealth and status, given that they are commonly made of precious metals and stones.
Tiger's eye is a chatoyant gemstone that is usually a metamorphic rock with a golden to red-brown colour and a silky lustre. As members of the quartz group, tiger's eye and the related blue-coloured mineral hawk's eye gain their silky, lustrous appearance from the parallel intergrowth of quartz crystals and altered amphibole fibres that have mostly turned into limonite.
Onyx is the parallel-banded variety of chalcedony, an oxide mineral. Agate and onyx are both varieties of layered chalcedony that differ only in the form of the bands; agate has curved bands while onyx has parallel bands. The colors of its bands range from black to almost every color. Specimens of onyx commonly contain bands of black and/or white. Onyx, as a descriptive term, has also been applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".
A brooch is a decorative jewellery item designed to be attached to garments, often to fasten them together. It is usually made of metal, often silver or gold or some other material. Brooches are frequently decorated with enamel or with gemstones and may be solely for ornament or serve a practical function as a clothes fastener. The earliest known brooches are from the Bronze Age. As fashions in brooches changed rather quickly, they are important chronological indicators. In archaeology, ancient European brooches are usually referred to by the Latin term fibula.
A ring is a round band, usually made of metal, worn as ornamental jewelry. The term "ring" by itself denotes jewellery worn on the finger; when worn as an ornament elsewhere, the body part is specified within the term, e.g., earrings, neck rings, arm rings, and toe rings. Rings fit snugly around or in the part of the body they ornament, so bands worn loosely, like a bracelet, are not rings. Rings may be made of almost any hard material: wood, bone, stone, metal, glass, gemstone or plastic. They may be set with gemstones or with other types of stone or glass.
Precious coral, or red coral, is the common name given to a genus of marine corals, Corallium. The distinguishing characteristic of precious corals is their durable and intensely colored red or pink-orange skeleton, which is used for making jewelry.
The Thetford Hoard is a hoard of Romano-British metalwork found by Arthur and Greta Brooks at Gallows Hill, near Thetford in Norfolk, England, in November 1979, and now in the British Museum. Dating from the mid- to late-4th century AD, this hoard is a collection of thirty-three silver spoons and three silver strainers, twenty-two gold finger rings, four gold bracelets, four necklace pendants, five gold chain necklaces and two pairs of necklace-clasps, a gold amulet designed as a pendant, an unmounted engraved gem, four beads, and a gold belt-buckle decorated with a dancing satyr. A small cylindrical lidded box made from shale also belonged to the hoard.
The pectorals of ancient Egypt were a form of jewelry, often in the form of a brooch. They are often also amulets, and may be so described. They were mostly worn by richer people and the pharaoh.
The Street House Anglo-Saxon cemetery is an Anglo-Saxon burial ground, dating to the second half of the 7th century AD, that was discovered at Street House Farm near Loftus, in the unitary authority of Redcar and Cleveland, England. Monuments dating back as far as 3300 BC are located in the vicinity of the cemetery, which was discovered after aerial photography revealed the existence of an Iron Age rectangular enclosure. The excavations, carried out between 2005 and 2007, revealed over a hundred graves dating from the 7th century AD and the remains of several buildings. An array of jewellery and other artefacts was found, including the jewels once worn by a young high-status Anglo-Saxon woman who had been buried on a bed and covered by an earth mound.
The Snettisham Jeweller's Hoard is a collection of Romano-British jewellery and raw materials, found during the construction of a house in the Norfolk village of Snettisham in 1985. The hoard is thought to be the working stock of a jeweller, buried in a single clay pot around 155 AD. The finds include the working tip of a quartz burnishing tool, partially or fully completed items of jewellery, and raw materials: mainly silver coins, scrap silver items and silver ingots, but also six pieces of scrap gold, and many engraved gemstones to be set in rings. The presence of scrap gold and silver and absence of base metals indicates that the jeweller dealt mainly with high-status customers.
Ancient Roman jewelry was characterized by an interest in colored gemstones and glass, in contrast with their Greek predecessors who focused primarily on the production of high-quality metalwork by practiced artisans. Extensive control of Mediterranean territories provided an abundance of natural resources to utilize in jewelry making. Participation in trade allowed access to both semi-precious and precious stones that traveled down the Persian Silk Road from the East.