Also, the density, or specific gravity of different materials can have the same effect. A one carat sapphire for instance is much smaller than a one carat opal. When looking for a setting for a particular gemstone, the stones dimensions in millimeters should be used, not the carat weight. The term is sometimes used to approximate a quarter carat. Gram , unit of weight in the Metric system.
Inclusions are differnt from blemishs, which are imperfections at the surface of the gemstone.
Inclusions are not always bad. Inclusions can be used like a finger print, to identify diamonds and other valuable gemstones. Inclusions can often be used to tell the difference between natural and synthetic gemstones, and in some types of gemstones, the inclusion can even pinpoint the source of the natural stone. In some cases, inclusions can actually increase the brilliance of a gemstone. Beware of emeralds and rubies which do not contain inclusions, as these are extremely rare, and are probably synthetic.