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Agate: Mohs hardness: 6.5-7

Agate is a form of chalcedony that occurs in nodular masses in rocks such as volcanic lavas. Inside, agate reveals an amazing variety of colors and patterns and a distinct banding which distinguishes it from other types of chalcedony.

Amber: Mohs hardness: 2-3

Amber is an organic gem formed from the hardened, fossilized sap or resin of ancient pine trees. Amber often contains bubbles and other small inclusions such as small plant or animal fossils. Though its transparency and color varies, the most common colors are orange, yellow, and brown. It also occurs in green.

Amethyst: Mohs hardness: 7

Amethyst is the best known and most valuable form of quartz. Its color is caused by iron or manganese compound impurities in the mineral. The best quality comes from Russia and Brazil. Amethyst was supposed to have many useful powers in the 15th century, but its traditional virtue was its ability to cure drunkenness.

Aquamarine: Mohs hardness: 7.5-8. March birthstone

A variety of beryl meaning "sea water", which accurately describes its color. it has a range from pale green to blue, caused by varying amounts of different forms of iron. It is relatively common, with sources in many countries, Brazil being the main.

Beryl: Mohs hardness: 7.5-8

A transparent pale green, blue, or yellow mineral consisting of a silicate of beryllium and aluminum, sometimes used as a gemstone.

Carnelian: Mohs hardness: 6.5

Carnelian is a type of chalcedony that is a translucent reddish-orange. Its color comes from its iron oxide content and stones may present a uniform color or be faintly banded.

Chalcedony: Mohs hardness: 7

A variety of quartz that's composed of microcrystalline quartz crystals and hydrated silica (opal). Agate, jasper, carnelian and chrysoprase are all varieties.

Citrine: Mohs hardness: 7

Slow heating turns amethyst into citrine, a yellow variety of quartz. It is often mistakenly called topaz. citrine rarely occurs in natural deposits and is a valuable collectors item.

Diamond: Mohs hardness: 10. April birthstone

The word diamond is derived from the Greek word 'Adamas', meaning 'unconquerable', given to the stone because of it's supreme hardness. Diamond is made of pure carbon, each carbon atom is strongly bonded to four others to form a rigid, compact, immensely strong crystal structure. It is this which makes it the hardest of all minerals. Diamonds form in the Earth's mantle in conditions of extreme heat and pressure. At depths of about 93 miles they develop a compact atomic structure that gives them their hardness. Volcanic eruptions bring the diamond to the Earth's surface. Some stones may be as much as three billion years old. Diamonds were first discovered over 2000 years ago and came mainly from river gravel in India. In 1725 they were found in Brazil, which remained the major source until production in South Africa became significant in 1870. Today, about 20 countries produce diamonds. The top producer is Australia, which supplies a quarter of the worlds needs, mainly for industrial purposes. diamonds are used in a vast number of jobs mainly because they are so hard. They are used in sawing, drilling, grinding and polishing-from quarrying stone to performing delicate eye surgery, and come in a vast array of sizes , shapes and strengths. The most familiar consideration of the cut gemstone are what we know as the four "C"s; color, cut clarity, and carat. The most traditional cut is the "brilliant" with 57 facets. Diamonds are also available in every color, including black.

Emerald: Mohs hardness: 7.5-8. May birthstone

Emerald belongs to the beryl family and gets its green color from traces of chromium and vanadium. The finest examples of this precious stone come from Columbia, where they have been mined for many centuries. Many are mine and exported illegally and there are often robberies and murders connected to this stone.

Fire Opal: Mohs hardness: 5-6.5

These opals tend to have one body color and do not usually contain the flashes of light and color typical of other opals, it is the only natural opal that is normally faceted. Generally found as an intense red-orange color, hence the name.

Garnet: Mohs hardness: 7-7.5. January birthstone

Garnet is the name of a group of chemically related, silica based minerals that includes almandine, pyrope, spessartine, grosular, and andradite. They can all be used as gemstones, the almandine-pyrope group being the most widely used. Because of the different chemical compositions, garnet occurs in most colors, except blue. Sources of gem-quality material include Czechoslovakia, South Africa, USA, Australia, Brazil and Sri Lanka. The name garnet may have come from the Latin word used for pomegranate, 'granatum.' You can easily see the similarity of the colors of the glossy gemstone and the seeds of the fruit.

Hematite: Mohs hardness: 6-6.5

Hematite is iron oxide, a common iron ore found in igneous rock. Kidney-shaped lumps and shiny black crystals are both common forms of hematite. Hematite gemstones are opaque with iridescent surfaces.

Iolite: Mohs Hardness: 7-7.5

Iolite is the gem name for cordierite, a silicate of aluminum and magnesium. Iolite is sometimes called dichroite, alluding to its dichroic properties; it shows two or more colors according to the direction in which it is viewed through transmitted light. Most iolite is found in gravel beds in the form of water-worn pebbles.

Jade: Mohs hardness: 6-7

Jade is tougher than steel, but soft enough to be suitable for carving. The Spanish conquerors of Mexico believed that the Indian's green stones would cure kidney ailments. They called them kidney stones or 'piedras de hijades' and from this the word jade was derived. In Europe, the name was then given to material of the same color and hardness which was imported from China. It was first used as tools and weapons. Later it was made into jewelry and ornaments. In 1863, the French scientist Damour showed that two different minerals shared the name jade. "Jadeite" is composed of interlocking, granular pyroxene crystals. Nephrite is an interlocking aggregate of fibrous amphisole crystals. Colors range from white, cream, grey, soft to medium mint green, spinach green, dark olive green to near black.

Lapis Lazuli: Mohs hardness: 5.5

Lapis lazuli is not a single mineral but a rock consisting of blue lazurite with variable amounts of calcite and pyrite. The name is derived from the Persian word 'lazhward', meaning blue. The blue color is caused by sulfur which is an essential part of its composition. In medieval times lapis lazuli was crushed and purified to make the paint pigment ultramarine. Primarily mined in Afghanistan, other sources include the Soviet Union and Chile.

Malachite: Mohs hardness: 3.5-4

Copper minerals near the Earths surface alter when exposed to water and cool temperatures. Malachite often forms in these conditions. It is often found as Kidney-shaped masses surrounded by bands of color. It always has a green color which varies only in intensity. Malachite is widely used as a decorative stone. It is also a secondary ore of copper.

Moonstone: Mohs hardness: 6-6.5

The most familiar gem variety of feldspar minerals. The white or blue iridescent sheen is caused by layers of tiny crystals of albite within orthoclase and is called 'adularescence". Some varieties may be steely gray, orange pink, yellow, or pale green. The gray stones particularly may show good cat's eyes.

Opal: Mohs hardness: 5-6.5. October birthstone

Opal is a relatively soft semi-transparent solidified mineral composed of silicon and water. The popularity of opal has risen and fallen over the centuries. The ancient Romans used it as a symbol of power but since then, at different times, it has been considered to be unlucky. The Aztecs mined opal over 500 years ago in Central America. Australia is the top producer of both black and white opals; they were first discovered there in the 1870's. Opal is one of the few noncrystalline gems. It has a tendency to crack and chip, especially under extreme temperature changes or after a hard knock. the exciting flashes of color shown by precious opal are best displayed in cabochons.

Peridot: Mohs hardness 6.5-7. August birthstone

Peridot is a French word and may derive from the Arabic 'faridat', meaning a gem. It is the gem variety of a group of minerals that are found in igneous rocks called olivine, a magnesium and iron silicate that is common in volcanic rocks. This group only forms in molten rock beneath the Earth's surface. Peridot is yellowish to olive green with an oily luster. The finest peridot specimens come from the island of Zebirget in the Red Sea.

Quartz: Mohs hardness: 7

Quartz is a silicon dioxide. It occurs as individual crystals and fine-grained masses in a large variety of forms, patterns, and colors. It is widely distributed as veins and is associated with major mineral deposits. It is one of the chief materials in granite and is also the main component of sand and sandstone. As quartzite and sandstone, it is used extensively for building and in the manufacture of glass and ceramics. One of the most interesting properties of quartz crystals is piezoelectricity, this is an electric charge produced by applying pressure to them, which gives quartz many uses and contributes to its value.

Ruby: Mohs hardness: 9. July birthstone

Ruby, a variety of corundum, is one of the world's most valuable gemstones. A ruby's color is due to a trace of chromic oxide; the amount of this substance determines the depth of color. Because of their hardness and durability, rubies are an excellent choice for all types of jewelry. Star rubies asterisk is caused by needle-like inclusion of rutile.

Sapphire: Mohs hardness: 9. September birthstone

Sapphire is a member of the corundum family. Blue sapphire takes its color from titanium and iron oxides; the depth of color depends on the amount of oxides present. Star sapphires are available in blue and black. White sapphires are sometimes diamond substitutes. Some of the finest sapphires occur in river gravel in Kashmir. They originally formed in rocks beneath the Himalayas. Other colors are yellow and pink.

Tanzanite: Mohs hardness: 6-7

A purplish blue gem variety of the mineral zoisite. It was found in northern Tanzania in 1967 and is remarkable for its displays of rich blue, magenta, and yellowish grey. Many crystals are greenish grey and are heat treated to the more attractive blue. It is trichroic- it often exhibits different colors when seen from different directions.

Tiger's Eye: Mohs hardness: 7

Tiger's Eye is a variety of quartz with fine, similarly oriented fibrous inclusions. Tiger's Eye first develops with tiny fibers of silky blue asbestos. These were then dissolved by solutions which deposited quartz and iron oxides in their place. This gives rise to the light reflection know as chatoyancy (changeable lustre) or "cat's-eye."

Topaz: Mohs hardness: 8. November birthstone

A common misconception is that all topaz is yellow; pure topaz is colorless, but the gem actually occurs in a broad range of colors including yellow, brown, pink, red, blue and green. Blue Topaz is demarcated from lightest to darkest by: American (or Maxi), Swiss and London.

Tourmaline: Mohs hardness: 7-7.5

Tourmaline comprises ten mineral varieties. The range of color seen in tourmaline is the greatest of any mineral. Even a single crystal can have several colors. Tourmaline is a mineral with a complex chemistry. It crystallizes as prisms with flat or wedge-shaped terminations. Every crystal has a different structure at each end, sometimes indicated by different colors. This gives tourmaline an unusual electrical property. If a crystal is gently warmed, one end becomes positively charged and the other negatively charged. Tourmaline is a pleochroic mineral. This means that it looks a different color when viewed from different angles. It forms in igneous and metamorphic regions. It also forms in mineral veins. Tourmaline can grow with other minerals such as beryl, quartz, and feldspar.

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