In gemmology and cut grading, depth refers to the vertical measurement of a faceted gemstone from the table facet to the culet, expressed in millimetres. It is one of the three principal dimensions — alongside length and width — recorded on laboratory grading reports issued by organisations such as the GIA. Beyond its role as a simple measurement, depth is a primary determinant of how light travels through a stone, directly influencing brilliance, scintillation, and the face-up colour appearance that buyers and dealers observe in practice.

Depth as a Proportional Measurement

Raw depth in millimetres acquires its greatest analytical value when expressed as a ratio. For round brilliant diamonds, the GIA calculates total depth percentage by dividing the depth by the average girdle diameter and multiplying by one hundred. This single figure encapsulates whether a stone's pavilion and crown proportions fall within ranges that promote efficient light return. Round brilliants typically perform best within a total depth percentage of approximately 59 to 62.5 per cent; stones cut shallower or deeper than this range tend to leak light through the pavilion or trap it in ways that diminish brightness and introduce unwanted optical effects such as the dark central zone colloquially known as a nail head or the circular shadow called a fish eye.

For fancy shapes — ovals, cushions, pears, marquises — no single universally accepted ideal depth percentage exists, because outline ratios and faceting arrangements vary considerably among cutters and market preferences. Laboratory reports for these shapes still record depth as a millimetre measurement and as a percentage, leaving proportional evaluation to the grader's or buyer's interpretation.

Depth in Coloured Gemstones

For coloured stones, depth carries an additional optical consequence that does not apply in the same way to diamonds: it modulates colour saturation. Because coloured gemstones absorb specific wavelengths as light travels through the crystal, a deeper stone presents a longer optical path and therefore appears more saturated — sometimes to the point of appearing dark or inky in the face-up view. A shallow stone may look washed out. Skilled cutters calibrate depth to balance saturation against brightness, a judgement that varies by species, origin, and the specific hue in question. Strongly dichroic or pleochroic species such as tanzanite or alexandrite add further complexity, since the orientation of the optic axis relative to depth affects which pleochroic colour dominates the face-up appearance.

Unlike the diamond trade, the coloured-stone market has no standardised ideal depth percentages. Proportions are instead assessed relative to the species, the cut style, and the individual stone's colour characteristics. Calibrated commercial goods — stones cut to standard millimetre dimensions for use in pre-made settings — are often cut shallower than ideal in order to maximise yield from the rough, a trade-off that experienced buyers account for when evaluating brilliance.

Recording and Reporting Depth

GIA grading reports for diamonds state depth in millimetres to two decimal places and express total depth percentage to one decimal place. The figure appears alongside table percentage, girdle thickness, culet size, and polish and symmetry grades as part of the proportions section of the report. For coloured stones, GIA and other major laboratories — including Gübelin, SSEF, and Lotus Gemology — record millimetre dimensions (length × width × depth) as a factual measurement rather than as a graded proportion, since no equivalent proportional grading system for coloured stones has achieved universal adoption.

Practical Implications for the Trade

Depth has direct commercial consequences. A stone with excessive depth carries more weight below the girdle than is visible to the wearer; buyers effectively pay for carat weight that contributes little to the face-up appearance. Conversely, an unusually shallow stone may appear larger per carat but sacrifice brilliance and, in coloured stones, saturation. Dealers routinely calculate the ratio of a stone's face-up area to its carat weight — sometimes called spread — as a practical check on whether depth is proportionate. A round brilliant diamond of one carat with ideal proportions measures approximately 6.5 mm in diameter; a stone of the same weight but excessive depth may measure only 6.1 or 6.2 mm across the table, a difference immediately apparent when set.