If you buy a gemstone because its beauty pleases you or you are interested in its purported metaphysical properties, that is reason enough. To ensure the price you pay is fair and reasonable, however, it makes sense to know how to evaluate the quality of the stone.
Gems can be evaluated using what is referred to as the Four C’s: carat weight, color, clarity, and cut.
First, it’s important to distinguish between Carat and Karat. Karat refers to the fineness of gold—how many of it’s 24 parts contain gold. 18K gold is 18 parts gold with the remaining 25% made up of less expensive metals like copper or silver.
Carat Weight refers to gemstone weight. One carat is equivalent to one-fifth of a gram.
Each gem mineral has a different density. Two different types of gems can have the same millimeter size but a different carat weight because of the difference in density. This is a good reason to always weigh gems—a stone that looks like a two carat blue sapphire will weigh more than a two carat blue spinel because the sapphire has a higher specific density.
For more information, check out some of the resources I've listed on my Inspiration page to learn more about gemstones.