What is a diamond gauge and how is it used?

A diamond gauge is a precision instrument that measures a stone's length, width, and depth in millimetres. These dimensions matter because they must correlate correctly and predictably with the stone's stated carat weight: a 0.50ct round lab-grown diamond should measure approximately 5.2mm in diameter, while a 1.00ct round sits closer to 6.5mm. For lab-grown diamonds under 0.30ct, millimetre sizing is the standard reference point rather than carat weight, since small weight differences are harder to perceive visually. IGI certification documents both the carat weight and the physical measurements, giving buyers a reliable cross-check against the grading report.