November Birthstone - Topaz

Physical and Chemical Properties

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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.

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