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!
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