Smaller are lighter in color
Always think in terms of "cost-per-carat"
Only compare stones of equal size, shape and overall quality
The most desirable color for tanzanite is usually not possible in stones under 2 carats.
Simply put, a carat is a carat is 1/5th of one gram. There is one tanzanite specific issue when it comes to carats. That is that due to their transparency, the smaller stones will tend to be fairly light in color. Since deep-dark-blue is the most valuable color for tanzanite, if you are looking for that color, you may have to limit your search to stones over two carats. The good thing is that tanzanite is relatively inexpensive in gemstone terms, so a top-quality two carat tanzanite would still cost substantially less than a sapphire of the same size. Other than that, the following carat rules apply to all gemstones:
Think in terms of "per-carat cost" rather than total cost. This is because price per carat varies depending on scarcity of stones, quality of a stone, popularity of a stone or cut, etc.
When shopping for set stones, beware of "total weight" labels. Always determine the weight of the stone before it was set.
Due to the variability of cost per carat factors, always compare stones of the same size, cut, and overall quality.
Remember that tanzanite stones under 1/2 carat are generally priced by millimeter size, not by carat weight.
Just because a carat is a carat, doesn't mean that one carat tanzanite is the same size as one carat anything else. Different stones have different densities and should only be judged amongst themselves. Even then, cut can have an illusory effect, so compare matching cuts as well.