Quartz & Zoisite

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What is quartz?
Varieties of quartz
Where is it found?

What is Quartz?

FRACTURE conchoidal
REFRACTIVE INDEX Low (glass-like appearance, which doesn't make it a good gem)

Quartz has no cleavage and fails by brittle, conchoidal fracture; the fracture surfaces have vitreous luster.

Quartz in thin section

Quartz images.

Movie showing quartz structure


SiO2 exists in several forms, the most common of which is low-temperature (alpha) quartz.
Quartz: pure SiO2 is colorless, not normally used as a gem.
    sometimes carved (quartz crystal)
    used in optical devices, etc. (like radio frequency oscilltors)
    can be doubly terminated
The most common gem varieties of quartz are amethyst and citrine.
    comes in different shades of purple (due to Fe impurity, commonly with zoned distribution):
      light purple
      reddish purple
    it is also set as a cabachon
    radiation damage and iron (Fe) impurity are necessary to produce amethyst color
      Fe - O charge transfer gives color: O-- <=> Fe4+
      Fe3+ in Si site -> Fe4+
    Other comments
    transparent, shades of orange
    color may be zoned
    the color is due to the presence of Fe, which is also the impurity present in amethyst.

    The term "ametrine" refers to bi-colored citrine/amethyst (junction is a peach color).

Smoky quartz
    smoky quartz is Al+++- bearing quartz that has been exposed to radiation (natural and unnatural)
      it removes an electron from O-- (i.e., a color center effect)
      if there is sufficient Al and radiation, the crystal can turn completely black! ("morian")
    Heat treatement restores clarity
    Also sometimes faceted
Rose quartz
    has rose-pink color
    the color is often due to the element Titanium (Ti)
    used for beads etc.
    sometimes you can find star rose quartz which has inclusions
    often massive (aggregates of crystals, not single crystals)
Green quartz

Milky quartz

    milky appearance due to inclusions (often of fluid)
    often associated with gold deposits - used as a gem mainly if gold present
Rutilated quartz is (normally) clear quartz that contains fine, often oriented, rutile crystals.
    Rutilated quartz
    common in jewelry
    The term "venus hair" refers to fine red rutile needles
    Note the orientations of the rutile needles
Tourmaline and other inclusions
    examples of quartz containing tourmaline and Fe-oxide
    fine tourmaline needles
    a second example of green tourmaline in quartz
    other inclusions can involve minerals (possibly rutile) and open tubes, for example:
      at low magnification these features make interesting patterns,
      the three dimensional nature of these needles is quite apparent
Chatoyant quartz
due to the presence of needles of asbestos etc. Form when prexisting minerals are replaced by quartz ('fossilized')
(tiger's eye): brown/yellow, and red
if needles retain their green asbestos color: "hawk's eye"; "falcon's eye"... etc.
Other fibrous varieties, including agate and chrysophrase are discussed in later lectures

Adventurine: green quartz containing platy inclusions of mica; can be used as a substitute for jade; used in beads etc.

Quartz can also contain inclusions of trapped fluid that may contain mineral precipitates and gas bubbles ('fluid inclusions') [Note the bubble of liquid and gas in this image is round. The black line is marker to draw your attention to the appropriate region).]. These tell us about the fluids present when the quartz formed. For example, fluids may be quite saline, indicating quartz grew from salt-rich solutions.

Where does it come from ?

  • Quartz is a very common mineral in most igneous, metamorphic, and sedimentary rocks. It has also been found in some lunar rocks and meteorites.
  • Quartz forms in rocks of igneous origin (melts) only after other silicates have incorporated most other available cations.
  • Rocks that are more than about 47 percent (by weight) silicon dioxide contain quartz. The quartz content increases as the weight percentage SiO2 increases.
  • Quartz can be dissolved in hot water or steam and is thus transported from place to place in the Earth, being deposited by cooling of the transporting fluid or by release of pressure.
  • Because quartz is relatively resistant to mechanical abrasion, it is abundant in stream sediments, on beaches, and in wind-blown sands. Quartzite and sandstone are mostly quartz because this is the most abundant mineral that survives processes occurring at the Earth's surface.


  • Quartz was first synthesized in 1845 via a hydothermal method  > 1 lb. crystals
  • Amethyst can be synthesized; distinguished mostly by "twinning"
Twinning produces many separate crystals of a mineral in rotated orientation, but orderly arrangement.

Doped quartz (quartz containing an impurity such as cobalt (Co) may be synthesized to achieve bright colored materials.

Heat Treatment: Citrine and amethyst:

Primitive methods can be employed like using a wheelbarrow to immerse crystals in sand within a fire to heat them to a high enough temperature to modify the oxidation state of the Fe.

Quartz has some interesting properties

some other comments

Zoisite (Variety Tanzanite)

What is it?
Where is it found?

What is it ?

  • Tanzanite is the important gem variety of zoisite (zoisite is also refered to as epidote)
  • Color is variable. Gem varieties are often sapphire blue or violet blue. However, other colors occur, like the red variety (thulite), and a green variety.
  • Hardness: 6.5-7 (note this hardness is less than normally desirable for a gem used in jewelry subject to abrasion by dirt and dust)
  • Crystal system: orthorhombic
  • Composition: Ca-Al-silicate (tanzanite color is attributed to a Vanadium (V) as an impurity
  • strongly pleochroic
  • View an uncut crystal.

View some images of tanzanite

Where is it found?

  • The variety of zoisite known as tanzanite was discovered in Tanzania in 1967
    • it occurs in pegmatite veins (inclusions suggest hydrocarbon-rich fluids)


  • usually heat treated at 400 - 500 C to remove brown tints, deepen blue
  • some cats eye varieties due to channels or needles
    • best orientation for color not same for chatoyancy

Previous Lecture:  Chrysoberyl, Rutile and Spinel
Next Lecture:  Olivine, Cordierite and Feldspar

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