How Calibrated Gemstones Are Specified

Standard commercial steps run from about 1.0 mm for round melee in quarter-millimetre increments, moving to half-millimetre steps for ovals and cushions from roughly 4 × 3 mm upward. Specify length × width for anything that is not round.

Order matters and is worth stating explicitly: 5 × 3 mm and 3 × 5 mm should mean the same stone, but making it unambiguous costs nothing.

Two stones with identical face-up dimensions can differ substantially in depth, and therefore in weight, in how they sit in a mount, and in whether the culet pokes through the back of a thin setting.

Cutters retain weight by leaving stones deep, because stones are sold by the carat. If you do not set a depth ceiling, you should expect to receive the deep end of the range.

Always state a maximum depth. It is the single most valuable line in a calibrated requirement, and the one most often left out.

For micro-pavé and channel setting, girdle thickness and consistency matter as much as outline. A knife-thin girdle chips during setting; an over-thick one will not seat properly.

If stones are going into machine-made settings, say so. It changes what a supplier will select from a parcel.

A repeating order is a different commitment from a one-off. The second delivery has to match the first, which means the colour band has to be recorded and worked to rather than judged fresh each time.

The practical way to handle this is a retained reference: keep a small set of stones from the first delivery, and have the supplier match subsequent runs against those rather than against a description. Colour language drifts; a physical reference does not.

Tell your supplier at the outset that the order repeats. It affects what they will commit to and what they will hold back.

Availability of calibrated goods varies sharply by species. Sapphire, aquamarine and peridot calibrate reliably; spinel does not, at any scale. Ask before you design around a species.