Precious topaz is a birthstone for November and blue topaz is a birthstone for December. Blue topaz is the gem of the 4th anniversary and Imperial topaz is the gem of the 23rd anniversary.
Topaz actually has an exceptionally wide color range that, besides brown, includes various tones and saturations of blue, green, yellow, orange, red, pink, and purple. Colorless topaz is plentiful, and is often treated to give it a blue color. Topaz is also pleochroic, meaning that the gem can show different colors in different crystal directions.
Red is one of the most sought-after topaz colors and represents less than one-half of 1 percent of facet-grade material found. The color the trade calls imperial topaz is highly prized and very rare. Many dealers insist that a stone must show a reddish pleochroic color to be called imperial topaz.
A fashioned topaz that displays a combination of two colors is called bicolor topaz. Some say that pink topaz, often called rose topaz, resembles a pink diamond or a bright pink sapphire. Pink topaz has certain advantages over these two gems. It’s much less expensive than pink diamond, and it’s often available in larger sizes than either diamond or sapphire.
Dealers often use the trade term “sherry topaz” for yellowish brown or brownish yellow to orange topaz. The term comes from the color of sherry wine. Stones in that color range are also sometimes called precious topaz. This helps distinguish them from less expensive citrine and smoky quartz, both of which look similar to, and are frequently misrepresented as, topaz.
Golden or yellow topaz lacks the prized red overtones of imperial topaz. It’s also much more abundant and therefore less valuable. Although brown topaz is also less valuable, it has been used in striking pieces of jewelry and ornamental art.
In nature, topaz is most commonly colorless, and naturally strong blue gems are extremely rare. In the marketplace, however, strong blue shades are plentiful. Treatments are the reason for this. Treaters use a combination of radiation and heat to produce blue hues in topaz. Since the 1970s, treatments have brought blue topaz to a broad market.
Faceted blue topaz is almost always free of eye visible inclusions. Other more rare colors like imperial and pink may show inclusions more often and still be valuable due to the color’s rarity.
Read more on http://www.gia.edu/topaz