TOPICS

1. What is topaz?
2. Where is topaz formed?
3. Geographic localities
4. Treatment of topaz

(1) Topaz

How are topaz crystals cut?

• crystals: up to 100 lbs in weight !!
• world's largest facetable gem (22,982 ct)
• appearance: vitreous (glassy luster)
• color:
  • reddish-yellow ("imperial")
  • orange-brown ("sherry")
  • clear
  • blue
  • pinkish
  • natural and treated blue (see below)

(2) Geologic conditions of formation:

Topaz crystallizes from fluorine-bearing vapor in last stages of solidification of igneous rocks. Thus, cavities in lavas and granitic rocks:

• in pegmatites
• in alluvial deposits

(3) Geographic location:

• Brazil: Ouro Preto and Minas Gerais
• Siberia and the Ural Mnts.
• Pakistan (esp. pink topaz)
• Sri Lanka (esp. irradiated topaz)

(4) Treatments:

IRRADIATION AND HEAT TREATMENT

Topaz is the most common irradiated gem on the market. Particles, or electromagnetic rays (ionizing radiation), have enough energy to produce color centers. High energy particles include:

• electrons
• alpha particles
• beta particles
• gamma particles: Co-60 Gamma Cell
• neutrons

Details:
• alpha particles = high speed helium (He) atoms with no electrons
• beta particles = high speed electrons
• gamma rays = high energy photons of electromagnetic radiation (similar to x-rays, but shorter wavelenght and thus, more energetic)
• neutrons = neutral subatomic particles
• Irradiation is most often carried out in Linear accelerators (high energy electrons) and Nuclear reactors (high energy neutrons)

• residual radioactivity potentially a problem. Stone may have to be stored to allow radioactivity to decay (half life may be hours to days)

• Nuclear reactors create an intense blue (that does not need secondary heat treatment)

• gamma rays => brownish greenish color + HEAT (see below) -> steel grey/blue ("cobalt")

marketing names: "sky", "london", "swiss" , "super", "cobalt", "max", etc.

Some examples:

• very light blue
• light blue
• medium blue
• medium-dark blue
• deep blue

Irradiation is used to change the color of many gems!

EXAMPLES OF COLOR MANIPULATION:

HEAT ONLY:

orange brown Cr-bearing topaz + HEAT (450 C) and cool -> pink-purple (termed 'pinking')

Why ?

• color = sum of: Cr -> pink and a color center -> yellow-brown
• recall, a color center is often an electron trapped in an atomic site, a form of structural damage often caused by radiation
• Heat allows crystal to heal itself and release electron from trap, removing brownish tinge (leaving pink)

HEAT AFTER IRRADIATION:

topaz irradiation -> greenish brown color (= blue + yellow + reddish tinge)

each of the components of the color is due to a color center. Least stable are the yellow and reddish; controlled heating removes these, leaving blue color centers.

WHO CARES?

Treatment is controversial because:

1. swamp out naturally colored material, reducing its value
2. irradiation can not be detected
3. color may not be stable
4. safety concerns (rarely a problem)

How much "radiation is associated with gemstones?

How does this source compare with the major sources of radiation exposure to a human being? Continued next lecture!

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Synthesis

occasional

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SPECIALLY FOR STUDENTS:

Further explanation of the key concepts

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View a collection of topaz images

Questions on the lecture

"Beryl"

"Zircon"

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