When looking at the carat weight of a gemstone, you should keep in mind that the weight does not necessarily allow you to properly envision the actual size of the gemstone.
Because different gemstones have different densities or specific gravity, two gemstones that appear to be the same size may actually have very different weights. In the jewelry industry, it is common to refer to millimeter size and then convert to carat weight. It is very important to ask about the dimensions of a gemstone to make sure that the majority of the weight of the stone will be seen when set. If putting in the setting you would base it off the millimeters of the stone, not the weight.