| Topaz
is a fluorosilicate of aluminum having the relatively simple formula Al2(F,OH)2SiO4.
It is a birthstone for November (as is citrine) and the state gem of Texas. Topaz is found
throughout the world in certain silicic volcanic rocks and in pegmatites.
Perhaps no other gem name has been, or continues to be,
so widely misapplied. The confusion dates back to its very origin (from the Greek
topazos), which derives from the name of an island in the Red Sea that was, at one time,
an important source of peridot! In ancient times the name was applied to all yellow
stones, regardless of their other characteristics. In more recent times, the name has been
(and continues to be, in some countries) applied to citrine or smoky quartz, sometimes
(but not always) with the modifying adjectives "Smoky", "false",
"semiprecious", "quartz", etc. Most reputable jewelers in this country
now regard such a practice as unethical (see F.T.C. ruling of 1967),
yet it is still a practice that see widespread use in certain circles and in certain
countries; caveat emptor!
Topaz varietal names based on color are
limited to one,
Imperial
Topaz, which connotes a vivid reddish-orange, but is also sometimes used
(incorrectly) to denote a sherry-colored,
orangish-brown.
Other colors are blue,
pink, red
(very rare), lavender (also very rare), greenish-yellow, yellow, brown and colorless.
Colorless and blue varieties contain higher concentrations of fluorine than do yellow or
pink stones, resulting in variations in the optical and physical properties. Except in
some red or pink stones, where trace quantities of Cr act as a chromophore, the color in
topaz is due to color centers. This makes them good candidates for color-change through
irradiation and heating. Most natural topaz is colorless or
very pale blue; the dark blue color so
commonly seen today is produced by irradiation, usually followed by heating.
Properties
- Crystal System: Orthorhombic
- Habit: Usually well-formed prisms, with a basal
pinacoidal termination on one end and a small pinacoidal face surrounded by pyramid and
horizontal prism faces at the other.
- Hardness: 8
- Cleavage: 1; perfect, perpendicular to c axis.
- Toughness: fair; somewhat brittle due to cleavage.
- Specific Gravity: 3.5-3.6
- pink - 3.50-3.53 (3.53)
- yellow - 3.51-3.54 (3.53)
- colorless - 3.56-3.57 (3.56)
- blue - 3.56-3.57 (3.56)
- red - 3.49-3.57 (3.53)
- R.I.: Usually 1.61-1.64; reported range is 1.607-1.64
- pink, yellow, red - 1.62-1.63
- colorless and blue - 1.61-1.62
- Birefringence: Low (0.005-0.009)
- Dispersion: Low (about 0.014)
- Pleochroism: weak to none, except in pink stones. Some
yellow varieties may show a weak yellow to pink dichroism.
- Color: see above.
- U.V. Fluorescence: weak; blue and colorless topaz may
show a weak yellow or greenish glow under long wavelength u.v.. Sherry-brown, pink, or
golden yellow topaz may show a strong orange-yellow fluorescence under long wavelength
u.v. light that is much weaker under short wavelength u.v. light.
Distinguishing
Properties
- R.I.
- Hardness (simplest way to distinguish from citrine or aqua.)
- Cleavage
- Relatively high S.G.
- Lack of pleochroism
Sources
Requires a source of fluorine; during the late-stage
crystallization of certain types of igneous rocks fluorine vapor is present and can be
trapped in pegmatites or gas cavities in volcanic rocks. In the latter setting, topaz is
often associated with tin deposits. In pegmatites it is often associated with apatite,
tourmaline, fluorite, and beryl.
- Brazil (Minas Gerias, near Ouro Preto)
- Most of the worlds production; from pegmatites
-
Imperial Topaz (worlds only source), sherry-colored, yellow and blue
- Sherry-colored is heated to produce pink
- Most (if not all) Brazilian blue topaz has be treated (see below) to enhance or alter
the original color.
- Mexico, near San Luis Potosi
- Significant source; in cavities in rhyolitic volcanic rocks
- Most very pale pink to colorless, or sherry-brown
- Some more deeply colored stones are known to fade in sunlight
- Sri Lanka
- Mined from same gem gravels that produce corundum, beryl
- Mostly pale to dark yellow, colorless or pale green
- U.S.; minor production
- Fine blue from Pikes Peak, CO (pegmatites)
- Pale brown to colorless, Thomas Range, Utah
(volcanic rocks)
- Pale blue to colorless, Mason Co.(near Streeter), TX (pegmatites)
- Also scattered pegmatites in NH, ME, SC, CA
- Others: Pakistan (pink), Nigeria (colorless), USSR (blue, pale green, rose), Zimbabwe
(blue), Germany (yellow), Australia (colorless), Burma (yellow, blue, colorless)
Shaping and
treatment
- Step or brilliant cuts. Perfect cleavage needs to be oriented at
an angle to table in order to polish effectively (very difficult to polish cleavage
surface).
- Radiation + heat treatment of blue stones ubiquitous (see below).
Very few naturally deep blue stones.
- Some pink stones obtained by heating brown, yellow or orange
material (Cr and possibly some Fe? must be present for this to work). Though poorly
documented, some or most(?) of these stones are said to have been irradiated as well.
- Topaz jewelry should not be ultrasonically cleaned; the vibration
can cause incipient cleavage cracks to open, cleaving the stone.
Color Enhancement of
Blue Topaz
Blue topaz is currently the biggest selling
"nontraditional" colored stone in the U.S.. The current popularity of blue topaz
is a direct result of its phenomenally low cost compared to other blue stones (aquamarine,
sapphire) which, in turn, can be traced to the veritable glut of blue topaz that has
resulted from the widespread
treatment of abundant colorless or pale blue Brazilian topaz to a darker blue color. A
variety of techniques
have been and are currently being used to enhance color in topaz. All involve the creation
(and sometimes partial destruction) of color centers. The most widely used techniques are
described below. It is important to realize that none of the techniques produce
"characteristic" colors that can be ascribed to a particular treatment, nor are
the trade names used to describe the treated material always definitive. There is, in
fact, no definitive method for differentiating among stones treated by the various
techniques or for telling treated from untreated stones.
- 1) Gamma Cells - exposure to a gamma ray
source (60Co commonly used) will produce both blue and yellow color centers in
some topaz, resulting in a brownish or brownish-green color. Subsequent heating removes
the less stable yellow color centers without affecting the blue. Such treatment, though
once common, is not widely used today because it is not capable of producing the darker,
purer blues that are in highest demand and that can be created by the other techniques
described below. The light blue color produced by gamma ray treatment has been given the
trade name "Cobalt Blue". Darker blue stones that have undergone such a
treatment are often grayish blue ("steely" blue). Gamma ray treatment is used
more often today as a "pretreating" or screening process that can reveal stones
that are more likely to be susceptible to treatment by the higher energy techniques
described below; such stones will take on a bluish tint during this initial treatment.
- 2) Linear Accelerators ("linac"
treatment) - topaz exposed to high energy electron beam. Process generates considerable
heat; topaz is water cooled to avoid thermal shock and spontaneous destruction of color
centers. Process must be followed by heat treatment, as above, to destroy the unwanted,
less stable yellow color centers. Result is typically a deeper blue stone (with little to
no gray; trade name "Sky Blue") than can be produced by gamma ray treatment, but
one that is radioactive for a short period of time. Such stones must be allowed to sit for
a couple of weeks in order for the residual radiation to decline to safe levels.
- 3) Nuclear Reactors - topaz exposed
to fast neutrons, producing blue color. No subsequent heating required. Color is typically
a medium to dark grayish blue, sometimes described as "steely" or
"inky". Trade name for this material is "London Blue". Heat treatment
can be used to lighten the inky cast. Material treated this way is likely to be quite
radioactive and may require several months or more of storage before the radioactivity
decays to safe levels. All material treated this way falls under the jurisdiction of the
Nuclear Regulatory Commission; all producers and importers of this material must be licensed by the
NRC.
- 4) Combination Treatments -
treatments combining nuclear reactor, linear accelerator and heating to produce dark blue
colors without the "inkiness" of the London Blue material. Trade names for this
material include "Electra Blue", "Super Blue", "New Blue",
"Swiss Blue", "Max Blue", "American Blue", "California
Blue", and "Super Sky Blue".
Pricing and valuation
- Flawless or nearly flawless topaz rough widely available; don't accept a stone that is
visibly included.
- Reds, intense pinks or violets are rarest, most expensive (reds $300-$1000/carat; pinks
$100-$500/carat in 1-5 carat sizes)
-
Imperial topaz of a very intense orangish-red next most expensive ($100-$400/carat in 5
carat sizes; $50-$200 in 1-2 carat sizes)
- Yellow to yellow-brown is common, less than $50/carat
- Blue, particularly deep blue ("London blue"), extremely popular, common; has
replaced aqua. as the "inexpensive" blue stone of choice because it costs
considerably less. Recent wholesale prices
for 3 carat gemstones:
- Sky Blue - $0.75-1.50/carat
- London Blue - $3-4.50/carat
- Super Blue - $3.50-10.00/carat
- Some treated stones (particularly the expensive reds, pinks, violets and deep oranges, but
not blue) are known to fade with prolonged exposure to direct sunlight.
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