A 2-millimeter round diamond is approximately 0.03 carats, or 6 milligrams. Other cuts and various types of stones have greater weights. Colored stones generally have a larger volume when cut and weigh more than an equally sized diamond.
The density of a stone affects its weight as well. For example, a 2-millimeter round amethyst is 0.04 carats, and a 2-millimeter round ruby is 0.05 carats. A 2-millimeter square or princess-cut diamond is 0.06 carats. While 2 millimeters is always just under 1/12 of an inch, the carat weight of a precious stone depends on the shape and type of gem.