White (or clear) is the most common color of topaz. The colorless variety generally has the least value, but stones can sell for $80 or more, depending on size and quality.
Topaz is mined in clear, yellow, brown, blue and pink varieties. Clear is the most common, but yellow and brown are found in abundance as well. Blue and pink are less common and therefore the most expensive.
Gemstones are priced based on four qualities: color, clarity, cut and carat. As for colorless gemstones, there are three grades of colorless and four grades of white distinguished by subtle variations of transparency. Clarity judges inclusions, or impurities, in the stone—basically anything that interferes with the passage of light. Cut is one of the most important facets of pricing a gemstone, but it's the most difficult to judge. Carat isn't as cut and dried as one would think. Larger stones are more rare and therefore command a higher price per carat than smaller stones.
White Topaz Pricing
As of July 2010, white topaz is priced at $10 per carat for a fair quality stone. A good quality stone is priced between $10 and $30 per carat. A very good quality stone is priced between $30 and $80 per carat. An excellent quality stone is priced at $80 per carat and up.