Chemical composition -- Aluminum oxide.
Color -- almost all colors. Medium to dark red stones are known as ruby; other colors, sapphire.
Optics -- R.I. 1.76-1.77.
Durability -- Hardness 9. After diamond, the next hardest gem material. Tough, but still subject to chipping and abrasion. Stones worn regularly in rings will periodically need repolishing to remove minor chips and pits.
Crystal structure -- Hexagonal .
Specific Gravity -- 4.0.
Sources -- Many locations, but the best known are in southeast Asia, Australia, and southern Africa. In the U.S., Montana and North Carolina are well known sources.
Wednesday, September 30, 2009
Quartz
Chemical composition -- Silicon dioxide.
Quartz is one of the most abundant minerals on Earth and is found in many different forms almost everywhere. There are two major classifications of quartz: crystalline quartz and cryptocrystalline quartz.
Color -- Very wide color range -- colorless, yellow, brown, purple, pink, greenish. Cryptocrystalline quartz often displays exotic color bands, swirls, and other patterns.
Optics -- R.I. 1.553-1.554. Uniaxial positive.
Durability -- Hardness 7.
Crystal structure -- Hexagonal.
Specific Gravity -- 2.651 for crystalline material, up to 2.91 for cryptocrystalline.
Sources -- Extremely widespread.
Quartz is one of the most abundant minerals on Earth and is found in many different forms almost everywhere. There are two major classifications of quartz: crystalline quartz and cryptocrystalline quartz.
Color -- Very wide color range -- colorless, yellow, brown, purple, pink, greenish. Cryptocrystalline quartz often displays exotic color bands, swirls, and other patterns.
Optics -- R.I. 1.553-1.554. Uniaxial positive.
Durability -- Hardness 7.
Crystal structure -- Hexagonal.
Specific Gravity -- 2.651 for crystalline material, up to 2.91 for cryptocrystalline.
Sources -- Extremely widespread.
Rutile
Chemical composition -- Titanium oxide.
Color -- Brownish-red to black.
Optics -- R.I. 2.62-2.90. Metallic to adamantine luster.
Durability -- Hardness 6-6.5.
Crystal structure -- Tetragonal.
Specific Gravity -- 4.2.
Sources -- Brazil, George, Arkansas, North Carolina, other locations. Usually found as needle-like crystal inclusions in quartz (rutilated quartz) or agate (sagenite) and in many other materials.
Color -- Brownish-red to black.
Optics -- R.I. 2.62-2.90. Metallic to adamantine luster.
Durability -- Hardness 6-6.5.
Crystal structure -- Tetragonal.
Specific Gravity -- 4.2.
Sources -- Brazil, George, Arkansas, North Carolina, other locations. Usually found as needle-like crystal inclusions in quartz (rutilated quartz) or agate (sagenite) and in many other materials.
The Garnet Family
Chemical composition -- The garnets comprise a family of complex silicates with widely varying chemical composition but similar structures. While everyone is familiar with dark brownish or purplish red garnets, many are unaware that garnets can occur in almost any color except blue (although there have been some recent reports of some color change garnets that are predominantly blue). Slight variations in chemical composition define the placement of a garnet within the family.
Optics -- R.I. varies with composition, but generally 1.74-1.94. Isometric.
Durability -- Hardness 6.5-7.5.
Crystal structure -- Isometric (cubic).
Specific Gravity -- 3.4-4.2
Varieties -- Many garnets are mixtures of the primary varieties and show variations in properties according to the specific composition.
Optics -- R.I. varies with composition, but generally 1.74-1.94. Isometric.
Durability -- Hardness 6.5-7.5.
Crystal structure -- Isometric (cubic).
Specific Gravity -- 3.4-4.2
Varieties -- Many garnets are mixtures of the primary varieties and show variations in properties according to the specific composition.
Amber
Chemical composition -- An organic gem material, amber is the fossilized resin of pine trees that lived some 30 million years ago. Copal is a more recent fossil resin; its properties are very similar, but it is more readily attacked by solvents.
Color -- usually shades of yellow, brown, or red.
Optics -- Singly refractive. R.I. about 1.54. Typically displays internal stress patterns under a polariscope.
Durability -- very soft, hardness about 2 to 2.5.
Crystal structure -- amorphous.
Specific Gravity -- very low, about 1.05-1.096. Amber will usually float in a saturated solution of salt water, unlike most of its simulants, such as plastic.
The thermal conductivity of amber is low, so it feels warmer to the touch than non-organic gem materials.
If touched by a hot needle, amber releases a sweet, resinous odor. In contrast, plastic substitutes have an acrid odor.
Amber is particularly prized when it contains intact fossilized insects. If the insects are large and well formed and the amber is attractively colored and relatively free from other faults, it can command substantial prices.
Sources -- include the region around the Baltic Sea and the Dominican Republic, but it occurs in many other locales.
Recent news reports indicate that one researcher says he has successfully revived numerous strains of ancient bacteria trapped in amber! The May 29, 1995, issue of Newsweek (p. 69) described how microbiologist Raul Cano of California Polytechnic State University claims to have revived 25 million year old Bacillus bacteria from the guts of bees trapped in ancient amber. Shades of Jurassic Park!
Color -- usually shades of yellow, brown, or red.
Optics -- Singly refractive. R.I. about 1.54. Typically displays internal stress patterns under a polariscope.
Durability -- very soft, hardness about 2 to 2.5.
Crystal structure -- amorphous.
Specific Gravity -- very low, about 1.05-1.096. Amber will usually float in a saturated solution of salt water, unlike most of its simulants, such as plastic.
The thermal conductivity of amber is low, so it feels warmer to the touch than non-organic gem materials.
If touched by a hot needle, amber releases a sweet, resinous odor. In contrast, plastic substitutes have an acrid odor.
Amber is particularly prized when it contains intact fossilized insects. If the insects are large and well formed and the amber is attractively colored and relatively free from other faults, it can command substantial prices.
Sources -- include the region around the Baltic Sea and the Dominican Republic, but it occurs in many other locales.
Recent news reports indicate that one researcher says he has successfully revived numerous strains of ancient bacteria trapped in amber! The May 29, 1995, issue of Newsweek (p. 69) described how microbiologist Raul Cano of California Polytechnic State University claims to have revived 25 million year old Bacillus bacteria from the guts of bees trapped in ancient amber. Shades of Jurassic Park!
Monday, August 31, 2009
Tourmaline is a real miracle of colour. It not only comes in green, red, blue, yellow, colourless and black, but also as a multi-coloured or colour-changing gemstone or as a cat's eye. There are, furthermore, innumerable mixtures of colour, in all nuances and depths, and some very unusual tones too. However, until recently, there were no pure yellows in the rich colour range of the 'gemstone of the rainbow', as this stone is also known. Most of the yellow tourmalines found thus far had a slight tinge of brown. But the tourmaline not only has many different colours; it is also good for a surprise now and then, as for example at the beginning of the 1990s, when some fantastic blue-green to turquoise tourmalines suddenly arrived on the market from a find in Paraiba, Brazil. Meanwhile, this colourful gemstone has taken the world by surprise again with another new variety, and this time it is a yellow one: in southern East Africa, in Malawi, a gemstone deposit with some wonderful yellow tourmalines was discovered in the autumn of 2000. The fresh, springlike yellow of these tourmalines is clear and pure and has just a very fine hint of green. Under the trade name 'canary', the new tourmaline variety has now begun to circulate. This is a particularly interesting kind of tourmaline. It is fine traces of magnesium that are responsible for the electrifying yellow colour. Since not all the raw crystals actually show that radiant yellow when they are found, some of the stones first have to submit to a period spent in the oven at approximately 700 degrees Celsius. Without this treatment, the colour would have a slight brownish tinge. The treatment only brings about the desired result because tourmalines typically display different colours and different colour intensities in different directions. By heat treatment, the tourmaline's second colour, in this case a light brown, is also transformed into the coveted radiant yellow. This is a kind of treatment customarily undertaken with many gemstones, the result of which is irreversible.Large yellow tourmalines are rare in Malawi too, the more so in view of the fact that only some 10 per cent of the yield is actually of gemstone quality at all. When cut, more than 95 per cent of the stones weigh less than one carat. However, they have wonderful wearing qualities, for like all tourmalines the canary-yellow beauties from Malawi have a good hardness of 7 to 7.5 on the Mohs scale. There is another speciality that distinguishes these gemstones from others, and that is their fine smell. It is an odour that the person wearing the stone will not notice, but the cutter will, while he is giving the raw crystals their final shape with his steady hand and taking great care that the fresh yellow is brought out to the best possible advantage. Experienced cutters are particularly fond of working on these gemstones, and say that these are the only gemstones that smell good. So why does a gemstone smell? The explanation is simple: tourmaline crystals are often embedded at the place where they are found in a black material which needs to be removed before cutting begins. One day, the owner of the gemstone mine in Malawi discovered that the unwanted black material was easier to remove if the raw crystals were first boiled in water to which lemon juice had been added. So since then, the yellow tourmaline crystals from Malawi have had not only the pleasant colour of fresh lemons, but also their fragrance. At least up to the moment when cutting starts.
Opals
All of Nature’s splendour seems to be reflected in the manifold opulence of fine Opals: fire and lightnings, all the colours of the rainbow and the soft shine of far seas. Australia is the classical country of origin. Almost ninety-five per cent of all fine opals come from the dry and remote outback deserts.Numerous legends and tales surround this colourful gemstone, which can be traced back in its origins to a time long before our memory, to the ancient dream time of the Australian aborigines. It is reported in their legends that the creator came down to Earth on a rainbow, in order to bring the message of peace to all the humans. And at the very spot, where his foot touched the ground, the stones became alive and started sparkling in all the colours of the rainbow. That was the birth of the Opals.The group of fine Opals includes quite a number of wonderful gemstones, which share one characteristic: they shine and sparkle in a continually changing play of colours full of fantasy, which experts describe as “opalising”. Depending on the kind, place of occurrence, and colour of the main body, we differentiate Dark or Black Opal, White or Light Opal, Milk or Crystal Opal, Boulder Opal, Opal Matrix, Yowah Nuts from Queensland – the so-called “picture stones“, and also Mexican and Fire Opal. Opal variations are practically unlimited. They all show in their own special way that unique play of colours – except for Fire Opal, which due to its transparency, however, is nevertheless also considered a Fine Opal specimen. If Opals are lacking the typical play of colours, they are simply named “Common Opal”.
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