Weighing every melee diamond individually would take days. Sieving a parcel takes minutes and gives every stone a size class buyers everywhere understand. This guide covers how diamond sieves work, the technique sorting houses actually use, and how to build a sieve setup that matches your parcels.
What is a diamond sieve and how does it work?
A diamond sieve is a pair of stacked pans: a top plate punched with precisely sized round holes, and a solid catch pan beneath. Stones smaller than the hole diameter fall through; larger stones stay on top. Run a parcel through a sequence of plates and every stone ends up in a size class defined by two plate numbers - larger than the plate it sat on, smaller than the one before.
What do sieve plate numbers mean?
Plate numbers refer to hole diameter, and the trade quotes melee sizes as plate ranges: a stone that passes plate 12 but sits on plate 11 is a +11-12. Because round brilliant proportions are standardised, each plate range corresponds closely to a carat weight per stone, which is how a sieved parcel converts to per-stone weights without weighing each diamond. A full sieve set covers the melee range in fine steps; individual loose plates let you extend a set or replace a worn plate without buying everything again.
How to sieve a parcel: the working technique
- Start clean. Dust and oil slow stones through the holes. Keep plates clean and stones dry; a washing jar earns its keep here.
- Work from large to small. Stack or run plates in descending order so each pass removes the largest remaining stones.
- Shake with rhythm, not force. A steady horizontal swirl lets stones present themselves to the holes. A sieve stand or shaker makes long sessions faster and more consistent than hand shaking.
- Give each pass enough time. Stop when stones stop falling, not before; rushing a pass is the main source of mixed classes.
- Verify by weight. Weigh each class on a carat scale and divide by stone count. The per-stone average should match the plate range; if it does not, re-run the class.
- Record and pack. Sort classes into parcel papers or gem boxes labelled with the plate range.
Economical, mid-range or premium sieves: what changes?
Punching precision and consistency. Premium plates are punched to tighter hole tolerances and stay round through years of daily use, which is why sorting houses running volume choose premium sieve sets. For occasional sorting, an economical set classifies accurately enough for buying decisions; mid-range sets balance the two. Whatever the tier, plates are consumables: inspect for out-of-round holes yearly and replace individually from loose plates.
Common sieving mistakes to avoid
- Overloading the plate. A crowded plate hides holes; sieve in batches shallow enough that every stone can reach the surface.
- Mixing shapes. Sieves assume round stones; fancy shapes classify by width, not weight, and need separate handling.
- Skipping the verification weigh. Plate wear and rushed passes both show up instantly in a per-stone average check.
- Dirty plates. Oil films change effective hole size; clean plates classify honestly.
The bottom line
A sieve set, a stand, a carat scale and clean technique will size thousands of stones an hour with repeatable accuracy. Browse the complete diamond sieves collection - full sets, loose plates and stands, trade quality, stocked in Bangkok and shipped worldwide since 2001.