Buyers should be aware that gemstones are often sold in calibrated sizes. This means that stones are sold in common sizes that will fit most commercial jewelry, such as rings and bracelets. However, don't be afraid of a stone with an unusual size if you are working with a jeweler on a custom piece. A jeweler can easily size any piece to match your stone.
Mass-produced gem settings come in standard sizes. If a gem won't fit in a standard setting, one needs to be custom made. This involves much more labor and therefore expense. Bear this in mind when buying a gem, or selecting a cut for a gem. There are circumstances where the value of a stone makes it worth cutting for maximum yield, then later paying for a custom-made setting. In other circumstances, the cost of a custom setting isn't justified by the value of the gemstone.
A gem does not need to be the precise size of the setting. A skilled jeweler can modify a setting to accommodate a wide variety of gem sizes. Any round gem can be put in a standard setting. With other shapes, there are various factors to be considered. As a rule, gems under one carat can be within .1 mm of the setting size. Over one carat, a .2 mm variance can usually be accommodated and sometimes more.
The weights given are for diamonds cut to ideal proportions. Colored stones are usually cut with greater volume than diamonds; therefore, they will weigh more than the chart sizes. The density of the material will also affect the weight. For example a 6.5 mm round diamond, sapphire and opal, all cut in the same proportions will all have different weights. The diamond (with a specific gravity of 3.52,) will weigh 1 carat. A sapphire (specific gravity 4,) will weigh 1.14 carats. The opal (specific gravity 2.15) will only weigh .61 carats. There are subtle variations to this list. For example, some list a .25 carat diamond as measuring 4.1 mm. The point is not to take these figures too literally, but to use them as a starting point for estimating weights.
Calibrated Gems for Standard Jewelry Mountings / Settings