Amethyst Geode     QUARTZ      Siberian Amethyst   Rio Grande Citrine

    Quartz is silicon dioxide, SiO2, the second most common mineral in the crust of the earth (less common than feldspar). Although found in abundance in nearly all rock types, gem quartz nearly always comes from igneous rocks, principally pegmatites and cavities in volcanic rock, or from hydrothermal veins. Quartz is the most diverse and abundant of all gem materials, a fact reflected in its low price and popularity among lapidaries throughout the ages. Amethyst, carnelian and other colored varieties (described below) were among the first materials to be fashioned by early lapidaries for adornment and, like many other gems, were once highly prized for presumed medicinal and talismanic attributes.

    Gemologists recognize two broad categories of quartz, coarsely crystalline and finely (or crypto-, meaning hidden) crystalline. Coarsely crystalline varieties, like amethyst, citrine, smoky quartz and others (see below) occur as relatively large, single crystals that can be faceted or cut en cabochon. Cryptocrystalline quartz, called chalcedony (kal-sed'-nee), is composed of intergrown aggregates of microscopic or submicroscopic quartz crystals. Such materials, like agate, onyx, carnelian, and many others (see below) are commonly translucent to opaque (the many small crystals scatter light) and are thus usually cut en cabochon.

    The principle industrial use of quartz is in the manufacture of electronic devices (radio and TV tuners, watches, radar, sonar, etc.) that use quartz resonators to generate waves of a fixed frequency, or to convert resonance at a given frequency to a current. These unique applications are possible because quartz has no center of symmetry and thus exhibits a property called piezoelectricity. Pressure applied to a quartz crystal generates a weak electric current. Alternatively, an applied alternating current causes a crystal to resonate with a fixed frequency. High purity, untwinned crystals are cut into small thin plates for these applications. Large synthetic quartz crystals, grown by a hydrothermal process, are produced in enormous quantities to satisfy the industrial demand. The same process is used to grow amethyst, citrine and other colors of coarsely crystalline quartz, some of which are unknown or rare in nature (e.g. blue quartz), for gem application. Of greatest concern is synthetic amethyst, which can be extremely difficult to reliably distinguish from its natural counterpart, but can often be detected by a lack of twinning (polariscope), its flawlessness (microscope), and weak to absent color zoning (immersion liquid). All of these characteristics are uncommon in natural amethyst.


Properties

  • Crystal System: Hexagonal (rhombohedral)
  • Habit: Crystals commonly prismatic with rhombohedral terminations. Horizontal stria common on prism faces. Cryptocrystalline varieties are massive.
  • Hardness: 7
  • Fracture: Conchoidal
  • Cleavage: 1: Extremely poor
  • Specific Gravity: 2.6-2.65
  • R.I.: 1.54-1.55; Fine grained varieties as low as 1.53.
  • Birefringence: V. low
  • Dispersion: V. weak
  • Pleochroism: Usually v. weak; Amethyst - blue to violet, Smokey - pale to dark brown/ brown to black
  • Color: Crystals commonly strongly zoned parallel to rhombohedral faces, producing "chevrons" of darker and lighter color. Sector ("pinwheel") zoning about the c-axis produces alternating colored and uncolored regions that conform with rhombohedral terminations. Cryptocrystalline varieties show a wide range of allochromatic colors (see below).

Distinguishing Properties

  • Low R.I.
  • Low S.G.
  • Anisotropic
  • Color zoning in some varieties

Varieties

  • Coarsely Crystalline
    • Rock Crystal - colorless; low R.I., low dispersion, and lack of color make this a rather "common" appearing faceted gem.
      • Sometimes called "diamond" e.g. "Herkimer Diamonds", "Pecos Diamonds"
      • Carved into objects, spheres (e.g. crystal balls)
      • Major producer Brazil; U.S. near Hot Springs, AK, upstate, NY
      • Synthetic rock crystal produced in large quantities for industrial purposes.
    • Amethyst - shades of violet or purple quartz.
      • Color due to small amounts of Fe distributed in layers parallel to rhombohedral faces
      • Some strongly dichroic e.g. reddish violet to pale blue
      • "Siberian" = dark purple; "Rose-of-France" = pale purple to pink
      • Principle sources are Brazil and Uruguay, as crystals lining cavities in lava flows. Also found in pegmatite veins. Fine U.S. material rare; from Maine, New Hampshire, N. Carolina, Colorado, Wyoming.
    • Citrine -Pale to dark yellow, ranging to reddish orange
      • Color due to trace amounts of Fe
      • Nearly all citrine is produced by heating amethyst; retains color banding of amethyst but not pleochroism.
      • Some citrine produced by heating smokey quartz; often produces a greener yellow or green color.
      • Sometimes sold as "quartz topaz" or "topaz quartz" or "topaz"; Misrepresentation.
      • Distinguished from topaz by R.I., S.G. and color zoning.
      • Natural citrine is uncommon, found chiefly with amethyst.
    • Ametrine - quartz that is colored-zoned, containing both amethyst and citrine.
    • Smoky Quartz - black, brown, grading to citrine color.
      • Color due to radiation damage, color centers.
      • Heat treatment will transform to rock crystal.
      • Sometimes sold as "Smoky topaz" = quartz.
      • Very common material; best known source is Swiss Alps. Also in U.S. Pikes Peak CO, Maine, New Hampshire, Hot Springs, AK, Brazil.
    • Rose Quartz - pale pink to rose red.
      • Most is quite turbid, cloudy
      • Red color due to small amount of Ti.
      • Can contain aligned rutile needles to give 6-rayed star.
      • Cloudy material quite common; deep colored clear material rare.
      • Best is from Brazil, Madagascar, SW Africa, Maine, S.Dakota.
    • Milky Quartz - cloudy white
      • Most common of all coarse quartz varieties
      • Often in veins, pegmatites
      • Milky appearance due to fluid inclusions (trapped vapor or fluid)
    • Cryptocrystalline Quartz - material is an aggregate of a multitude of small crystals.  Two types of aggregates; fibrous (called chalcedony) and granular:
      • Fibrous - characterized by microscopic slender crystals
        • Chalcedony - s.s. honey yellow, waxy, translucent, slightly porous.
          • Sometimes dyed to many colors; e.g. "Swiss Lapis" or "German Lapis" is blue-dyed chalcedony.
          • Acid treatment also used to change color
        • Carnelian - red chalcedony (color from trace Fe)
        • Sard - brown chalcedony (color from FeOH)
        • Chrysoprase - apple green chalcedony (trace Ni)
        • Agate - Vary-colored chalcedony characterized by concentric banding of color. Formed by precipitation of SiO2 onto gas cavity walls in volcanic rock host.  A myriad of names describe different varieties; a few common ones are:
          • Moss Agate - dendritic growths of black MnO or reddish FeO in agate
          • Plume Agate - same, but forming plume-like shapes
          • Iris Agate - play-of-color
          • Fire Agate - Brown with fiery play-of-color
          • Petrified Wood - Chalcedony replacing plant material
        • Onyx - black and white layering in parallel planes; term also used for brown- and white-banded calcite i.e. "Mexican Onyx".
        • Sardonyx - onyx with red to orange layers alternating with white and black
      • Granular Varieties
        • Jasper - opaque, dull red (FeO); also green, dull blue, or black
        • Ribald Jasper - general term for banded Jasper
        • Chert - White, gray or black, glassy, and containing fewer impurities than jasper. Not usually a gem material
        • Plasma - nearly opaque, light green; used extensively in carving. Do not confuse with jade.
        • Prase - more translucent, darker green, than plasma; green due to mineral inclusions of chlorite and amphibole.
        • Bloodstone (heliotrope) -dark green with red spots of Fe2O3 or jasper. Once the birthstone for March.

    Pricing and valuation

        The least expensive of common gem materials. Facetted coarsely crystalline varieties are in the following ranges:

    • Top quality Amethyst (rich purple with red flashes) - $20-$40/ct.
    • Good quality Amethyst - $5-$15/ct.
    • Top quality Citrine (deep orange-yellow, clean) - $10-$25/ct. Good quality Citrine - $5-$10/ct.

Notes Index | Corundum | Beryl | Diamond | Pearl | Opal | Jade | Topaz | Tourmaline
Peridot | Garnet | Zircon | Spinel | Quartz | Metals | Review Notes | References | Home


1998
Updated 08/20/09
Comments and questions to helper@mail.utexas.edu
Department of Geological Sciences
The University of Texas at Austin