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November Birthstone - Topaz
Physical and Chemical Properties
Chemical composition
Topaz is an fluosilicate of aluminum Topaz has a specific gravity of about 3.50
to 3.57.
Light shades (pink, yellow, etc.) usually occupy the lower range and
darker shades (blues) usually occupy the higher range, as do colorless stones.
Many of the blue topazes are derived from colorless stones that have been
heat-treated so that is one reason why colorless stones have a higher specific
gravity. The refractive index of topaz ranges from about 1.610 to about 1.617 in
colorless and blue to about 1.630 to about 1.637 in yellow and brown stones. The
fairly low refractive index and the fairly high specific gravity are an unusual
characteristic as refractive index usually varies upward with the specific gravity.
The anomalous, low refractive index of topaz may be due to the presence of the
very large fluoride ion in the crystal lattice. Topaz is an extremely hard silicate
mineral containing fluorine and hydroxl and is formed by fluorine-bearing vapors
created during the crystallization of igneous rocks (rocks formed from a molten
state).
Colors
Topaz comes in many colors including pink, blue, lavender, yellow, orange,
orange-yellow, brownish yellow, yellow-brown, red, and colorless. Although most
commonly found as yellow or blue in color, topazes also may be transparent,
brown, and most rarely pink or red. Their colors can be unstable and change
according to temperature. "Brazilian rubies" or "burnt topazes" are often yellow
topazes that have been exposed to moderate heat to turn pink or red. Often
confused with citrine quartz, yellow topazes tend to have more orange or pinkish
tones. Irradiating clear topazes can create a blue tone in the gem.
Back to Topaz Birthstones Summary Page
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