If you buy diamonds above a third of a carat, weight is a perfectly good way to describe them. A 0.50 ct round brilliant of decent make is about 5.1 mm across, near enough every time, because proportions at that size are tightly conventional.
Below about 0.18 ct that relationship falls apart. A parcel of stones all weighing 0.010 ct can vary meaningfully in diameter depending on how deep they were cut, and diameter is the thing your setting cares about. A stone that is 0.05 mm too wide does not go into the seat. No amount of correct weight fixes that.
So the trade measures small goods by the hole they fall through.
What a sieve actually is
A diamond sieve is a shallow metal plate perforated with round holes of one precise diameter. Plates come in a numbered series, and they stack. Polished goods are poured onto the largest plate and shaken; what passes through drops onto the next plate down, and so on.
After a few minutes what you have is not one parcel but a column of parcels, each one containing stones that passed one plate and were held by the next. That pair of plate numbers is the size description.
Reading the notation
A parcel described as +6 −8 has been through the number 8 plate — the minus — and held on the number 6 — the plus. Every stone in it sits between those two diameters, roughly 1.50 mm to 1.80 mm.
The convention reads backwards to most people the first time. The rule to hold on to is: minus is the plate it fell through, plus is the plate that caught it. Minus is therefore always the larger number and always the larger hole.
When you see a single number with a minus in front — −2, the size this firm is built around — it means the goods passed through the number 2 plate and nothing below caught them. That is the bottom of the commercial range, around 0.80 to 1.00 mm.
Sieve to millimetre to carat
These are the working conversions for a well-proportioned round brilliant. Treat the weights as indicative: they move with make, and a deep-cut parcel will run heavier for the same diameter.
| Sieve | Diameter | Approx. weight | Stones per carat |
|---|---|---|---|
| −2 | 0.80 – 1.00 mm | 0.003 – 0.005 ct | 200 – 330 |
| +2 −4 (star) | 1.00 – 1.25 mm | 0.005 – 0.010 ct | 100 – 200 |
| +4 −6 | 1.25 – 1.50 mm | 0.010 – 0.015 ct | 65 – 100 |
| +6 −8 | 1.50 – 1.80 mm | 0.015 – 0.025 ct | 40 – 65 |
| +8 −10 | 1.80 – 2.10 mm | 0.025 – 0.035 ct | 28 – 40 |
| +10 −12 | 2.10 – 2.50 mm | 0.035 – 0.055 ct | 18 – 28 |
| +12 −14 | 2.50 – 2.90 mm | 0.055 – 0.080 ct | 12 – 18 |
Why the range matters more than the midpoint
Two suppliers can both sell you "1.5 mm melee" and deliver quite different goods. One sieves properly and ships +6 −8, so nothing in the packet is below 1.50 mm. The other sorts by eye around a target and ships a spread from 1.35 to 1.70, averaging 1.5.
On a pavé run, the second parcel costs you real money. The undersized stones rattle or sit low, the oversized ones will not seat, and your setter spends the morning sorting rather than setting. The parcel was cheaper. The job was not.
The value of sieve notation is that it is a range you can verify with a plate, not an average you have to trust.
Checking a parcel on arrival
You do not need a full sieve set to spot-check a supplier. A single plate at the plus size of the range you ordered is enough. Pour the parcel onto it and shake: if the goods are what the packet says, essentially nothing should fall through. Anything that does is undersized.
Do it once on a first order from a new supplier. It takes two minutes and it tells you more about how that house works than any certificate at this size ever will.
What to send us
If you know the sieve range, send it — that is the fastest possible inquiry and we can usually quote the same day. If you do not, send the millimetre size from your CAD file, or a photograph of the setting with a scale next to it, and we will identify the range for you.