A diamond sieve is a precision-manufactured mesh screen used to sort and grade small diamonds — collectively termed melee — by physical diameter. Sieve sets consist of a graduated series of circular frames, each bearing a wire or sheet-metal mesh with openings of a specific, tightly controlled dimension. When a parcel of mixed-size stones is passed through the stacked set, each diamond is retained on the first mesh whose aperture is smaller than the stone's girdle diameter, thereby segregating the parcel into discrete, commercially recognised size categories. Diamond sieves are indispensable in cutting centres, sorting houses, and gemmological laboratories wherever uniformity of melee parcels is required.

Construction and Tolerances

Professional-grade sieves are manufactured from stainless steel or brass frames fitted with woven wire mesh. The critical variable is the aperture size: reputable manufacturers hold tolerances to within a few hundredths of a millimetre, since even small deviations can misclassify stones into the wrong commercial bracket. Frames are typically 75–100 mm in diameter and are designed to nest or stack concentrically, with the finest mesh at the base and progressively coarser meshes above. A solid catch-tray sits beneath the finest sieve to collect any stones that pass through all screens.

Mesh openings correspond to standardised melee size designations used across the diamond trade. Common reference points include:

• 0.80 mm aperture — the smallest commercially significant melee, averaging approximately 0.005 ct per stone
• 1.30 mm — corresponds to the widely traded "1.3" size, averaging roughly 0.01 ct
• 1.70 mm — approximately 0.02 ct average
• 2.00 mm — approximately 0.03 ct average
• 2.50 mm — approximately 0.05 ct average
• 4.00 mm — the upper boundary of most melee sieve sets, approaching the threshold at which individual grading becomes standard practice

The exact correspondence between aperture size and average carat weight assumes well-proportioned round brilliant cuts; fancy shapes or poorly proportioned stones may sieve differently from their nominal weight.

Method of Use

Sieves may be operated by hand or mounted in mechanical shakers. In high-volume sorting centres, motorised vibrating platforms agitate the stacked set for a fixed interval, ensuring that all stones find their natural resting level without operator fatigue introducing inconsistency. Hand-sieving, still common in smaller workshops, requires a controlled circular motion sustained long enough for every stone to settle. After sieving, the contents of each frame are tipped onto a white sorting tray for inspection and weighing.

It is standard practice to sieve a parcel at least twice — once in the normal orientation and once after reassembling the stack — to confirm that no stones have been retained on an incorrect mesh due to bridging or clumping. Moisture, grease, or polishing compound residue on stones can cause them to adhere to mesh wires and must be removed before sieving for accurate results.

Role in the Trade and Laboratory Practice

In the diamond manufacturing pipeline, sieving occurs at multiple stages: after sawing and bruting, to confirm that rough or semi-finished goods conform to planned size targets; after polishing, to verify finished melee before parcel assembly; and at the wholesale level, where buyers require certified size uniformity before pricing. Consistent sieve grades underpin the per-carat pricing structure of melee, since a parcel described as "2.0 sieve" commands a predictable average stone weight and therefore a predictable setting labour cost.

Gemmological laboratories, including the GIA, employ diamond sieves as part of quality-control protocols when handling large submissions of melee for grading or treatment detection. Uniform size groupings simplify the systematic examination of stones and reduce the risk of mixing parcels during processing.

Limitations

A sieve measures only the minimum girdle diameter — the dimension that determines whether a stone passes through a given aperture. Two stones retained on the same mesh may differ considerably in table diameter, depth, and therefore carat weight if their proportions diverge. For this reason, sieve grading is a practical commercial tool rather than a substitute for individual weighing where precise carat weights are required. Additionally, sieve sets do not assess cut quality, colour, or clarity; those attributes require separate evaluation.