Most agates occur as nodules in volcanic rocks or ancient lavas where they represent cavities originally produced by the disengagement of volatiles in the molten mass which were then filled, wholly or partially, by siliceous matter deposited in regular layers upon the walls.
Carnelian agathe
Carnelian, sometimes spelled cornelian, is a red or reddish-brown variant of chalcedony. The word is derived from the Latin word meaning horn, in reference to the flesh color sometimes exhibited.
Amethyst is a violet or purple variety of quartz often used as an ornament. The name comes from the Greek a ("not") and methuskein ("to intoxicate"), a reference to the belief that the stone protected its owner from drunkenness; the ancient Greeks and Romans wore amethyst and made drinking vessels of it in the belief that it would prevent intoxication. Because it has a hardness of 7 on the Mohs scale, amethyst is treasured for its use in jewelry. However, it is still just 1/15 as hard as diamond.
Chalcedony is one of the cryptocrystalline varieties of the mineral quartz, having a waxy luster. Chalcedony may be semitransparent or translucent and is usually white to gray, grayish-blue or some shade of brown, sometimes nearly black.
The carbonate mineral calcite is a chemical or biochemical calcium carbonate and is one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks, limestone in particular. It is also the primary mineral in metamorphic marble.
Citrine, also called citrine quartz or citrine topaz, is an amber-colored gemstone. It is a form of quartz with ferric iron impurities, and is rarely found naturally.
Jasper is an opaque, impure variety of quartz, usually red, yellow or brown in color. This mineral breaks with a smooth surface, and is used for ornamentation or as a gemstone. It can be highly polished and is used for vases, seals, and at one time for snuff boxes. When the colors are in stripes or bands, it is called striped or banded jasper.
Labradorite is a flashy stone. Its base color is usually gray, and when you turn it around, it displays flashes of other colors - such as blue, green, yellow, pink, purple, orange. Sometimes there will only be one color flash - but there can be more than one, too.
Lapis lazuli, also known as just lapis, is a stone with one of the longest traditions of being considered a gem, with a history stretching back to 7000 BC in Mehrgarh of Indian subcontinent, situated in modern day Balochistan, Pakistan.
Malachite is a carbonate mineral, copper carbonate hydroxide.. Individual crystals are rare, but do occur as slender to acicular prisms. Pseudomorphs after more tabular or blocky azurite crystals also occur. Malachite often results from weathering of copper ores and is often found together with azurite, goethite, and calcite.
Tiger's eye
Tiger's eye is a chatoyant gemstone that is usually yellow- to red-brown, with a silky luster. It is a fibrous silicified crocidolite (blue asbestos), a classic example of pseudomorphous replacement. An incompletely silicified blue variant is called Hawk's eye.
Onyx is a banded variety of chalcedony, a cryptocrystalline form of quartz. The colors of its bands range from white to almost every color (not including shades such as purple, blue, or black). Commonly, specimens of onyx available contain colors of white, tan, and brown.
Smoky quartz
Smoky quartz or Smokey quartz, also known as Cairngorm or Cairngormstone is a brown variety of quartz caused through the natural (or artificial) irradiation of aluminium-containing rock crystal. A very dark brown to black opaque variety is known as morion.
Quartz is one of the most common minerals in the Earth's continental crust and is made up of silica. A bed is a common form, particularly for varieties such as amethyst, where the crystals grow up from a matrix and thus only one termination pyramid is present. A quartz geode consists of a hollow rock (usually with an approximately spherical shape) with a core lined with a bed of crystals.
Morion quartz
Morion or morion quartz is a dark-brown to black opaque variety of smoky quartz resulting from the natural or artificial irradiation of aluminium-containing milk quartz.
Rose quartz
Rose quartz is a type of quartz which exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titanium, iron, or manganese, in the massive material. Some rose quartz contains microscopic rutile needles which produces an asterism in transmitted light.
Orthoclase is an important tectosilicate mineral, which forms igneous rock. It is also known as alkali feldspar and is common in granite and related rocks. Orthoclase is named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other.

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31 July 2012

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