• <rt id="wzsil"></rt>
  • <cite id="wzsil"></cite>

    <tt id="wzsil"><noscript id="wzsil"></noscript></tt>
  • <rt id="wzsil"></rt>

    1. Andalusite Value, Price, and Jewelry Information

      4.49-ct andalusite, medium-dark brownish green/orange-brown, round brilliant cut, 10.1 mm, Brazil. ? The Gem Trader. Used with permission.

      Andalusite

      Strongly pleochroic, andalusite can show shades of green, brown, and red when viewed from different directions. Although tough enough for most jewelry uses, this strikingly beautiful stone is largely unknown to the gem buying public.

      Andalusite Value

      The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.

      Andalusite Value via Gem Price Guide

      Accompanying value information:
      Top Color: YG 5/2 and O 5/4
      Faceted 1 to 4 carats 4 carats plus
      Top color to /ct to ,000/ct
      Medium color to /ct to /ct

      Andalusite Information

      Data Value
      Name Andalusite
      Varieties Chiastolite, Viridine
      Colors Pinkish, reddish-brown, rose-red, whitish, grayish, yellowish, violet, greenish, colorless.
      Crystallography Orthorhombic.?Crystals prismatic, striated, square in cross section. Massive, compact.
      Refractive Index 1.629-1.690
      Luster Vitreous to subvitreous.
      Hardness 6.5 - 7.5
      Wearability Very Good
      Fracture Even to subconchoidal
      Specific Gravity 3.13 - 3.17
      Birefringence 0.007-0.011. (Viridine: 0.029).
      Cleavage Distinct 1 direction
      Dispersion 0.016
      Heat Sensitivity No
      Luminescence None in LW. Brown fluorescence in SW (Lancaster, Massachusetts). Dark green or yellow-green fluorescence in SW (brown-green gems from Brazil).
      Luminescence Present Yes
      Luminescence Type Fluorescent, UV-Short
      Special Care Instructions None
      Transparency Transparent to opaque.
      Absorption Spectrum Deep green varieties from Brazil display Mn spectrum: knife-edge shadow at 5535, fine lines at 5505 and 5475; faint lines at 5180, 4950, and 4550.
      Phenomena Chatoyancy (cat's eye), very rare.
      Formula

      Al2SiO5 + Fe

      Pleochroism

      Strongly pleochroic; olive green to flesh-red (Brazil).?Usually yellow/green/red.?Blue andalusite from Belgium: blue/colorless/colorless.

      Optics

      α = 1.629 – 1.640; β = 1.633 – 1.644 γ = 1.638-1.650. Near-colorless andalusite reported at low end of this range; green material at upper end. Viridine: 1.66-1.69. Biaxial (-), 2V = 73 – 86°.

      Optic Sign Biaxial -
      Etymology

      After Andalusia, Spain (but see “Sources” below). Chiastolite is from the Greek chiastos, “arranged diagonally,” because the pattern of carbon inclusions in the gem resembles the Greek letter chi, which is written “X.” Viridine alludes to the viridescent or “greenish” color of this variety.

      Occurrence

      Metamorphic rocks, usually slates and schists as a contact mineral, or developed within mica schist or gneiss. Also as a detrital mineral and very rarely in pegmatites and granites.

      Inclusions

      Veils and rutile needles (common); liquid inclusions, hematite flakes (Brazilian material); carbonaceous inclusions (chiastolite).

      andalusites - Brazil

      Andalusite: Brazil (7.55, 2.40, 2.92, 9.55). Photo ? Joel E. Arem, PhD, FGA. Used with permission.

      Comments

      Andalusite is polymorphous with kyanite and sillimanite. This means these gemstones share the same chemistry but have different crystal habits. Although andalusite occurs abundantly as a mineral, material transparent enough to facet occurs quite rarely.

      andalusites

      Andalusites. Photo by cobalt123. Licensed under CC By-SA 2.0.

      Andalusite has very distinctive and attractive?pleochroism. This gem can show up to three different colors (trichroism) depending on the viewing angle and the gem’s cut and orientation. Sometimes, andalusites can even show multiple colors from one viewing angle.

      Gem shapes with a long axis such as ovals, marquise, or emerald cuts tend to show one color near the center and a second, usually darker color, near the ends. Square and round cuts usually blend the colors into a mosaic.

      Sometimes, lapidaries cut these stones to show pink and almost colorless shades. Others cut to display green in the center of these stones, with browns or various other combinations on the tips, depending on the rough orientation before cutting.

      Poorly cut and polished stones may appear dull. As Type II clarity grade gems, andalusites usually contain inclusions.?However, a large, eye-clean, well-cut andalusite demands attention. With a hardness of 6.5 to 7.5, it could find a home in almost any type of jewelry setting.

      pear-cut andalusite

      3.01-ct andalusite, eye clean, medium-dark yellowish green/reddish brown, pear brilliant cut, 14.3 x 6.9 mm, Brazil. ? The Gem Trader. Used with permission.

      Varieties

      Cat’s Eyes

      Fibrous inclusions in andalusites may create a chatoyant effect. Lapidaries can sometimes cab such stones to showcase this. However, such ?cat’s eye pieces are extremely rare.

      Viridine

      This grass to deep green variety of andalusite contains manganese.

      andalusite - viridine

      Grass-green to brown viridine crystals on matrix, 4.5 x 2.7 x 2.3 cm, Recruta Farm, Vitoria da Conquista, Bahia, Northeast Region, Brazil. ? Rob Lavinsky, www.iRocks.com. Used with permission.

      Blue Andalusite

      Andalusites from Ottré, Belgium have a blue color due to an Fe+2-Fe+3 charge transfer mechanism.

      Chiastolite

      Chiastolites are gray crystals with black, carbonaceous inclusions in cruciform patterns in their interior.?Due to these impurities, chiastolite has a lower hardness and specific gravity?(SG) than other andalusite varieties. Since it’s opaque, this variety is cut more or less as a curiosity. Typically, cross sections of chiastolite may show ?well-formed black crosses on a gray background. However, occasionally lapidaries cut these specimens into different shapes, such as spheres.

      Handmade chiastolite jewelry from Laraquete, Bío Bío Region, Chile. Photo by Mar del Sur. Licensed under CC By-SA 4.0.

      You may find chiastolites for sale as trapiche andalusites. The trapiche phenomenon in andalusites is currently being studied.

      Identifying Characteristics

      With its trichroism and red, green, and brown colors, andalusite may seem easy to distinguish from other gemstones. However, tourmalines have a similar hardness, color range, SG range, and moderate to strong pleochroism as well. Too casual observations of pleochroic colors can lead to confusion between these two different species of gems. (On the other hand, andalusites have a biaxial optic character, while tourmalines have a uniaxial optic character).

      In fact, clever cutting can mimic andalusite’s strong pleochroism. For example, a predominantly brown synthetic quartz piece with a shallow green layer near its periphery was cut to show off both the colors, as real andalusites might. Again, more careful observation will distinguish the real item from a simulant.

      faceted andalusite

      Faceted andalusite. Photo by Casse-cailloux. Public Domain. (Cropped to show detail).

      Synthetics

      Scientists have synthesized andalusite crystals via the hydrothermal method for research purposes. However, there is no known jewelry use for this material. If this gem becomes more well known, perhaps that will change.

      Enhancements

      Andalusites may rarely receive?heat treatments to improve color.

      Sources

      Andalusite is eponymous with Andalusia, a region in southern Spain. This gem received this name from the French mineralogist?Jean-Claude Delamétherie in 1798, after its supposed source. However, the material he examined came from Castile-La Mancha, a region squarely in the center of Spain. In fact, andalusite’s chiastolite variety had already been described by the Spaniard?José Turrubia in 1754. Nevertheless, the name “andalusite” stuck, and so did references to Andalusia as the type locality. Although Andalusia does produce colorless andalusites, the type locality should be noted as?El Cardoso de la Sierra, Guadalajara, Castile-La Mancha, Spain.

      Currently, Brazil is the main source of these gems. They can be found as pebbles in stream beds or on hillsides under layers of clay.

      Other notable gem sources include the following:

      andalusite - minas gerais Brazil

      Large andalusite crystal, Jenipapo Mine, Taquaral area, Minas Gerais, Brazil. ? Rob Lavinsky, www.iRocks.com. Used with permission.

      Stone Sizes

      Gems from Brazil can reach 75-100 carats. However, gems from most localities usually range from 1 to 5 carats. Andalusites in the 5 to 10-carat range cost several times more per carat than smaller stones. Stones over 10 carats are quite rare. Those over 20 carats are still?rarer.

      Care

      Although resistant to scratching due to its hardness, andalusite has distinct cleavage and brittle tenacity. ?Therefore, ring stones should have protective settings to avoid blows.

      Before subjecting any rare andalusites to mechanical cleaning systems, have a gemologist examine them and identify any inclusions they may have. While andalusites have a “Very Good” wearability score, inclusions of liquids or other minerals could shatter if heated or vibrated, thus damaging your gems. In the meantime, to be safe, use a soft brush, mild detergent, and warm water instead.?Consult our gemstone jewelry cleaning guide for more recommendations.

      andalusites - artistic colored stones

      Andalusites. Photos courtesy of Barbara Smigel, Artistic Colored Stones.

      by Joel E. Arem, Ph.D., FGA, Donald Clark, CSM IMG, Barbara Smigel, PhD. GG

      Enter your email to access the Ultimate Opal Buying Guide for free!

      FREE

      THANK YOU!

       

      PLEASE CHECK YOUR EMAIL FOR THE E-BOOK

      Follow us on Facebook

      亚洲人日本人jlzzy