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16.04: Beryl

  • Page ID
    3273
  • Beryl
    Chemical composition Be3Al2(SiO3)6Beryllium aluminum silicate
    Crystal system Hexagonal
    Habit Prismatic
    Cleavage Very difficult in one direction, rarely seen
    Fracture Conchoidal
    Hardness 7.25-7.75
    Optic nature Uniaxial -
    Refractive index 1.577-1.583
    (+ or - 0.017)
    Birefringence 0.005-0.009
    Dispersion Low, 0.014
    Specific gravity 2.72
    (+0.18,- 0.05)
    Lustre Vitreous to resinous
    Pleochroism Weak to moderate

    Beryl is a beryllium aluminum silicate that occurs in every color of the rainbow. When green, it's usually called emerald but there exists green beryl, which is not entitled to be called emerald because its coloring agent is different. When blue, it's aquamarine and when pink, morganite. Yellow is heliodor and colorless is goshenite.
    A rare raspberry red variety found in Utah is called bixbite. There is a very rare and costly variety termed Riesling beryl, that can be described as pale green colored with a warm, golden yellow flash. Two unusually dark blue types of beryl have been found as well: Maxixe beryl and True Blue Beryl. Maxixe beryl fades with exposure to light, True Blue Beryl does not.
    Beryl has been used as a physician's tool and for gazing stones since ancient times. Those beliefs persist today. Beryl is metaphysically attributed with the ability to cure a number of intestinal and stomach ills, such as nausea, ulcers, and seasickness.

    Characteristics

    The optical and physical data of beryl can vary between varieties and localities.
    Beryl belongs to the beryl group. Pezzottaite (IMA approved in 2003) is also a member of the beryl group.

    Genesis

    Metamorphic rocks in pegmatites

    Localities

    Beryl is found in many localities, among them being Brazil, India, Africa, Columbia, Australia, and Pakistan.

    Habit

    Prismatic with pyramidal and/or pinacoidal terminations. Often vertically striated.

    Physical data

    Mohs hardness: 7.25 - 7.75 (emerald is brittle).
    Specific gravity: 2.7 to 2.9, depending on variety.

    Optical data

    Refractive index: nε = 1.56 nω = 1.59, depending on variety.
    Birefringence: 0.004 to 0.009, depending on variety.
    Optical sign: uniaxial negative.
    Dispersion: low, 0.014.
    Pleochroism: weak to moderate, strong in "True Blue Beryl" and pezzottaite.

    Diaphaneity

    Transparent to opaque.

    Colors

    All varieties of beryl are allochromatic. Main coloring agents are given below.

    • Green (emerald), colored by chromium 3+
    • Green (vanadium beryl), colored by vanadium 3+ - Also known as vanadium emerald
    • Green (green beryl), colored by ferric 3+ and ferrous 2+ iron
    • Green/Yellow (Reisling beryl), unknown coloring agent (most likely iron)
    • Blue (aquamarine), colored by ferrous 2+ Iron
    • Blue (Maxixe), colored by color centers
    • Blue (True Blue), colored by ferrous 2+ iron - Trade name for high FeO content aquamarine
    • Pink (morganite), colored by manganese 2+
    • Red (bixbite), colored by manganese 3+
    • Yellow (heliodor), colored by ferric 3+ Iron
    • Colorless (goshenite)

    Enhancements

    Common enhancements to beryl varieties:

    • Fracture filling (emerald) - oil, wax or plastic (with dye or without)
    • Coatings (emerald) - non-metallic paint or plastic (usually green)
    • Heat treatment - improves or even changes the color