CARNELIAN[MINERAL]

Listing description

Carnelian (also spelled cornelian) is a reddish-brown mineral which is commonly used as a semi-precious gemstone. 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

Detailed description

The color can vary greatly, ranging from pale orange to an intense almost-black coloration.

History

The bow drill was used to drill holes into carnelian in Mehrgarh between 4th-5th millennium BC.^[2] Carnelian was recovered from Bronze Age Minoan layers at Knossos on Crete in a form that demonstrated its use in decorative arts;^[3] this use dates to approximately 1800 BC. Carnelian was used widely during Roman times to make engraved gems for signet or seal rings for imprinting a seal with wax on correspondence or other important documents. Hot wax does not stick to Carnelian.^[4] Sard was used for Assyrian cylinder seals, Egyptian and Phoenician scarabs, and early Greek and Etruscan gems.^[5] The Hebrew odem (translated sardius), the first stone in the High Priest's breastplate, was a red stone, probably sard but perhaps red jasper.^[5]

Etymology

The word carnelian is derived from the Latin word caro, carnis meaning flesh, in reference to the flesh color sometimes exhibited.^[6] According to Pliny the Elder, sard derives its name from the city of Sardis in Lydia, but it more likely comes from the Persian word سرد sered, meaning yellowish-red.^[5]

Distinction between carnelian and sard

The names carnelian and sard are often used interchangeably, but they can also be used to describe distinct subvarieties. The general differences are as follows:^[5]

	Carnelian	Sard
Color	Lighter, with shades ranging from orange to reddish-brown.	Darker, with shades ranging from a deep reddish-brown to almost black.
Hardness	Softer	Harder and tougher.
Fracture	Uneven, splintery and conchoidal	Like carnelian, but duller and more hackley.

All of these properties vary across a continuum, and so the boundary between carnelian and sard is inevitably blurred.

Chalcedony (pronounced /kælˈsɛdəni/) is a cryptocrystalline form of silica, composed of very fine intergrowths of the minerals quartz and moganite.^[2] These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, whilst moganite is monoclinic. Chalcedony's standard Chemical structure (Based on the chemical structure of quartz) is SiO₂ (Silicon Dioxide).
Chalcedony has a waxy luster, and may be semitransparent or translucent. It can assume a wide range of colors, but those most commonly seen are white to gray, grayish-blue or a shade of brown ranging from pale to nearly black.
The name "chalcedony" comes from Latin calcedonius, the word used to translate the Greek word khalkedon, found only once, in the Book of Revelation; according to the OED a connection with the town of Chalcedon in Asia Minor is "very doubtful".^[3] There is no reason to assume that the precious stone referred to by this name in the Bible is the same as what is now understood by the name.^[4]

Varieties

Chalcedony occurs in a wide range of varieties. Many semi-precious gemstones are in fact forms of chalcedony. The more notable varieties of chalcedony are as follows:

Agate is a variety of chalcedony with multi-colored curved or angular banding. Fire agate shows iridescent phenomena on a brown background; iris agate shows exceptional iridescence when light (especially pinpointed light) is shone through the stone. Landscape agate is chalcedony with a number of different mineral impurities making the stone resemble landscapes.^[5]
Aventurine is a form of quartz, characterised by its translucency and the presence of platy mineral inclusions that give a shimmering or glistening effect termed aventurescence.
The most common colour of aventurine is green, but it may also be orange, brown, yellow, blue, or gray. Chrome-bearing fuchsite (a variety of muscovite mica) is the classic inclusion, and gives a silvery green or blue sheen. Oranges and browns are attributed to hematite or goethite. Because aventurine is a rock, its physical properties vary: its specific gravity may lie between 2.64-2.69 and its hardness is somewhat lower than single-crystal quartz at around 6.5.
Aventurine feldspar or sunstone can be confused with orange and red aventurine quartzite, although the former is generally of a higher transparency. Aventurine is often banded and an overabundance of fuchsite may render it opaque, in which case it may be mistaken for malachite at first glance.
The name aventurine derives from the Italian "a ventura" meaning "by chance". This is an allusion to the lucky discovery of aventurine glass or goldstone at some point in the 18th century. Although it was known first, goldstone is now a common imitation of aventurine and sunstone. Goldstone is distinguished visually from the latter two minerals by its coarse flecks of copper, dispersed within the glass in an unnaturally uniform manner. It is usually a golden brown, but may also be found in blue or green.
The majority of green and blue-green aventurine originates in India (particularly in the vicinity of Mysore and Madras) where it is employed by prolific artisans. Creamy white, gray and orange material is found in Chile, Spain and Russia. Most material is carved into beads and figurines with only the finer examples fashioned into cabochons, later being set into jewellery.
Main markets for aventurine are landscape stone, building stone, aquaria, monuments, and jewellery.
Carnelian (also spelled cornelian) is a clear-to-translucent reddish-brown variety of chalcedony. Its hue may vary from a pale orange, to an intense almost-black coloration. Similar to carnelian is sard, which is brown rather than red.

History

As early as the Bronze Age chalcedony was in use in the Mediterranean region; for example, on Minoan Crete at the Palace of Knossos, chalcedony seals have been recovered dating to circa 1800 BC.^[6] People living along the Central Asian trade routes used various forms of chalcedony, including carnelian, to carve intaglios, ring bezels (the upper faceted portion of a gem projecting from the ring setting), and beads that show strong Graeco-Roman influence.
Fine examples of first century objects made from chalcedony, possibly Kushan, were found in recent years at Tillya-tepe in north-western Afghanistan.^[7] Hot wax would not stick to it so it was often used to make seal impressions. The term chalcedony is derived from the name of the ancient Greek town Chalkedon in Asia Minor, in modern English usually spelled Chalcedon, today the Kadıköy district of Istanbul.
At least three varieties of chalcedony were used in the Jewish High Priest's Breastplate. (Moses' brother Aaron wore the Breastplate, with inscribed gems representing the twelve tribes of Israel). The Breastplate included jasper, chrysoprase and sardonyx, and there is some debate as to whether other agates were also used.
In the 19th century Idar Oberstein became the world's largest chalcedony processing center, in particular agates. Most of these agates were sourced in Latin America, in particular Brazil. Originally the agate carving industry around Idar and Oberstein was driven by local deposits that were mined in the 15th century.^[8] Several factors contributed to the re-emergence of Idar-Oberstein as agate center of the world: ships brought agate nodules back as ballast, thus providing extremely cheap transport. Cheap labor and a superior knowledge of chemistry allowing them to dye the agates in any color with processes that were kept secret.
Each mill in Idar Oberstein had four or five grindstones. These were of red sandstone, obtained from Zweibrücken; and two men ordinarily worked together at the same stone.^[8]

Geochemistry

Structure

Chalcedony was once thought to be a fibrous variety of cryptocrystalline quartz.^[9] More recently however, it has been shown to also contain a monoclinic polymorph of quartz, known as moganite.^[2] The fraction, by mass, of moganite within a typical chalcedony sample may vary from less than 5% to over 20%.^[10] The existence of moganite was once regarded as dubious, but it is now officially recognised by the International Mineralogical Association.^[11][12]

Solubility

Chalcedony is more soluble than quartz under low-temperature conditions, despite the two minerals being chemically identical. This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio.^{[citation needed]} It has also been suggested that the higher solubility is due to the moganite component.^[10]

Solubility of quartz and chalcedony in pure water

This table gives equilibrium concentrations of total dissolved silicon as calculated by PHREEQC using the llnl.dat database.

Temperature	Quartz Solubility (mg/L)	Chalcedony Solubility (mg/L)
0.01°C	0.68	1.34
25.0°C	2.64	4.92
50.0°C	6.95	12.35
75.0°C	14.21	24.23
100.0°C	24.59	40.44

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