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)

Geologic conditions of formation:

• in pegmatites
• in alluvial deposits

Geographic location:

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

Treatments:

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:

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)

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:

• Ready availability of artificial material could swamp out naturally colored material, reducing its value.
• Irradiation can not be detected.
• The color may not be stable; it may change over time.
• safety concerns (rarely a problem)

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

Further explanation of the key concepts