|Transparent clear topaz (sometimes called silver topaz) is quite common and has little value. It is cut into gemstones with good hardness (hence good durability) but topaz has perfect cleavage and can be a bit brittle, and can be easily broken when setting. It is very difficult to differentiate white topaz from rock crystal (quartz) in most cut stones.
Topaz also comes in many shades of yellow, orange, and a particular red-orange (called Imperial topaz). The most highly prized stones are the imperial topaz with a red flash and some of the "hot pink" topazes. These command the most value in todays market.
True imperial topaz comes from a limited number of localities in Brazil and contains some water in its structure. The structural water prevents the crystals from growing to large size in a clean state, and hence most imperial topaz natural crystals, that warrant cutting, are small or they are highly included and flawed.
Pale blue topaz has been mined, but today nearly all of the blue topaz is irradiated and heated to produce the varying shades of blue. Clear topaz or pale blue topaz can be irradiated and heated to intensify the blue color. Depending on the amount of colorant elements present the colors can range from very dark blue to very pale blue with most shades inbetween. There are a lists of names for different shades of blue topaz, but none are really official and there is little standardization. (london blue, sierra blue, sky blue, swiss blue, light blue, pale blue, etc.)
Blue topaz is a lovely color and a relatively inexpensive stone. It can be found in very large (100+ ct.) sizes and a wide variety of cuts. It is sometimes passed off as a poor mans aquamarine, and the two can be difficult to tell apart.
The brown to red-brown material is sometimes called Guerro topaz (also rootbeer topaz) and comes mainly from Mexico, South America and the western United States. Some of this topaz is not color stable and will loose its color on prolonged exposure to the sun. When very pale and brownish to tan in color it is sometimes called champagne topaz.
The straw yellow topaz is easily confused with pale citrine or pale smokey quartz, and these quartz stones have been sold under the names of "madera topaz" or "smokey topaz" in an effort to get more money for the stones.
Recently the market has seen an influx of a material called "mystic topaz", and most of this is topaz, but the wild coloration is a man-made phenomena. A thin layer of titanium oxide is placed on the colorless topaz base, and it produces a rainbow type effect. Titanium metal can be placed in a solution and controlled voltage can produce a wide variety of colors on its surface. The colors come from the surfac oxidtion and the disposition of layers of thin titanium oxide. (This is a common titanium and niobium jewelry making technique. See this site for more information: http://www.ganoksin.com/borisat/nenam/ajm-niobium-working.htm ) The "mystic topaz" gets its coloration from the same sort of oxide formation, but it uses physical vapor deposition of the titanium on to the the gem surface. The colors are on the surface and can be worn away.