01/10/2020
*Gems and gemstone*
Gem is a mineral or organic substance that is cut and polished and used as an ornament.
The qualities sought in gems are beauty, rarity, and durability. The unit of weight used for gems is the metric carat (200 mg).
Gems are usually cut to bring out their color and brilliance and to remove flaws.
Gems can be grouped into several categories using different criteria:
(a) The precious stones are diamond, some forms of corundum (e.g., ruby and sapphire) and emerald
(b) The chief semiprecious stones include amethyst, aquamarine, garnet, jade, moonstone, opal, quartz, topaz, tourmaline, and turquoise
(c) The organic gems are amber, coral, pearl, and jet
(d) The synthetic gems produced by using the chemical elements of natural stones and includes diamonds, emeralds, rubies, and sapphires
Popular gems The most popular and, therefore, the most important commercial gems include diamond, ruby, sapphire, emerald, and pearl.
The Diamond (at 10 on Mohs' scale, the hardest of all known substances) is a mineral and one of two crystalline forms of the element carbon.
It is the hardest substance known, and inferior stones are used as abrasives, in certain types of cutting tools, and as phonograph needles.
Gem diamonds were first found in stream beds in India and in Borneo; many are now extracted from volcanic pipes in South Africa,
Famous diamonds include the Koh-i-noor, now among the English Crown jewels; the Cullinan, from which 105 stones were cut; and the Blue Hope diamond.
Synthetic diamonds, produced since 1955, are now widely used industrially.
Though most highly prized when colourless, diamonds can be yellow, blue, pink, or other colours.
Corundum is an aluminum oxide mineral (Al2O3) occurring in both gem and in common varieties.
Ruby is coloured red by traces of chromium; the various colors of Sapphire, however, are produced by traces of iron and titanium in the corundum.
The ruby gem of the transparent red variety of corundum is found chiefly in Burma, Thailand, and Sri Lanka.
Star rubies (showing an internal star when cut with a rounded top) are rare. Synthetic rubies are produced by fusing pure aluminum oxide.
The sapphire gem is a transparent blue variety of corundum, found chiefly in Thailand, India, Sri Lanka, and Burma.
Like rubies, some sapphires show an internal star when cut with a round top.
Synthetic stones are made by fusing aluminum oxide, with titanium oxide added for color.
The transparent gems, chief of which are ruby and sapphire, are colourless, pink, red, blue (oriental aquamarine), green (oriental emerald), yellow, and violet.
Common varieties are used as abrasives (e.g., emery) and are blue-gray to brown in colour.
Corundum is found in North Carolina, Georgia, Montana, Republic of South Africa, and Tanzania.
The best gem corundum is found in Sri Lanka and elsewhere in Asia at places such as Mogok in Upper Burma, near Bangkok in Thailand, and Kashmir in India.
Beryl is an extremely hard beryllium and aluminum silicate mineral (Be3Al2Si6O18), occurring in crystals that may be of enormous size and are usually white, yellow, blue, green, or colourless.
Beryl gets its colour from traces of chromium and a little iron.
Beryl is the principal raw material for the element beryllium and its compounds.
It is commonly used as a gem, the most valued variety being the greenish emerald; the blue to bluish-green variety is aquamarine.
Aquamarine is a transparent blue to bluish-green variety of the mineral beryl and is used as a gem.
A typical jewellery store may carry several other kinds of gems: blue to bluish green aquamarine, pink to peach morganite, and other varieties (including those colored yellow green to green) of beryl; the complex borosilicate tourmaline, which may be pink to red (rubellite), blue (indicolite), bright green, or several other colors.
Tourmaline, complex aluminum and boron silicate mineral is also used as a gem.
Colors are red, pink, blue, green, yellow, violet, and black; sometimes it is colorless.
Two or more colors, arranged in zones or bands with sharp boundaries, may occur in the same stone.
Tourmalines are found in pegmatite veins in granites, gneisses, schists, and crystalline limestone.
The aluminosilicate topaz is straw- or golden-yellow, pink, green, blue, or brown in colour.
Topaz is an aluminum silicate mineral [Al2SiO4(F,OH)2], used as a gem.
Commonly colourless or some shade of yellow, the stone is transparent with a vitreous luster.
Topaz crystals occur in highly acidic igneous rocks and in metamorphic rocks.
Important sources include Brazil, Siberia, Burma, and Sri Lanka.
Gem varieties quartz (silicon dioxide) include amethyst (purple), chrysoprase (green), citrine (yellow to brown), rock crystal (colorless), and rose quartz; and Opal is a stone often with brilliant color flashes that is usually dark and white (Australia) or transparent (anywhere from colorless to orange-to-red and found in Mexico).
Opal, hydrous silica mineral (SiO2·nH2O) is formed at low temperatures from silica-bearing water, that can occur in cavities and fissures of any rock type.
Gem opal has rich iridescence and a remarkable play of colors, usually in red, green, and blue.
Most precious opals come from South Australia; other sources include Mexico (fire opal) and parts of the U.S.A as well as Sri Lanka.
The name garnet [Fe,Mg,Ca,Mn)3(Al,Fe,Cr)2(SiO4)3], is applied to a group of silicate minerals.
It is used chiefly as gems and abrasives.
The most common gem varieties are red, but garnets are also yellow, brown, and green. They are found in many types of rock throughout the world.
Less common gems include the beryllium aluminate chrysoberyl, especially its cat's-eye variety and the fascinating alexandrite variety, which appears green in daylight but appears red in artificial incandescent light; garnet, such as the red Bohemian pyropes and Indian almandines; peridot, a yellow green variety of the mineral species olivine, a magnesium-iron silicate; the magnesium-aluminate spinal, in its red and mauve varieties; tanzanite, a sapphire blue variety of the mineral species zoisite, a calcium-aluminum hydroxy silicate; and spodumene, in its green to yellow green varieties or the beautiful lavender lilac kunzite variety.
Turquoise in a hydrous aluminum and copper phosphate mineral Al2(OH)3PO4.H2O+Cu.
This usually found in microscopic crystals, it is opaque with a waxy luster, varying in color from greenish gray to (GEM-quality) sky blue.
Because of their porosity, the gem varieties absorb dirt and grease, changing the color to an unattractive green; exposure to heat or sunlight can also harm the color.
