A jewellery designer in Amsterdam sent us a CAD for a tension-set engagement ring. The render looked clean. We cut the seat to the depth on the drawing, set the 1.2 ct lab-grown stone, and on first light the pavilion showed a small star crack at the girdle. The seat had been cut 0.15 mm too deep, and the ring's spring pressure had split the stone on setting. Tension settings are unforgiving in a way prong settings are not: there are no prongs to bend back, only pressure. Here is how we cut seats so the stone locks without stressing it.
How a tension seat actually holds
A tension ring flexes open when you pry the shank, drops the stone into two opposing grooves, and clamps shut. The stone is held purely by the inward spring of the metal. That means the seat does two jobs at once: it locates the stone laterally, and it controls how much metal presses on the girdle. A seat cut to the full girdle thickness allows maximum clamp; a seat cut to 80% of girdle thickness leaves a ledge that reduces pressure and protects the stone. We never cut a tension seat deeper than 70% of the stone's girdle thickness, because that turns a clamp into a press.
We compare this against tension versus bezel durability, match the seat to the stone in girdle fitting for moissanite, and hold our tolerances in girdle tolerance for setting. Buyers can reference stone grading through GIA's 4Cs and cut-grade guidance when specifying tension-set diamonds.
Our seat-cutting routine
Measure the actual stone first
We never cut to the spec sheet. We measure the girdle thickness of the exact stone arriving for setting, with digital calipers, and cut the seat to 70% of that measured number. A 5% girdle variation between two stones from the same parcel is normal, and a seat cut to average will either be loose on the thick one or cracked on the thin one.
Undercut for the pavilion
We cut a small undercut below the seat so the pavilion does not bottom out on metal. If the pavilion touches the bottom of the groove, the stone sits higher than designed and the clamp never reaches the girdle. The undercut is 0.3 mm wider than the pavilion at the seat level.
Test-set with a dummy
Before the real stone touches the ring, we cut a resin dummy of the same dimensions and set it. The dummy should hold with a firm grip and not fall out when the ring is turned upside down. If the dummy rattles, the seat is too wide; if it will not drop in, the seat is too narrow. Only after the dummy passes do we set the real stone.
| Stone size | Seat depth | Undercut | Dummy test |
|---|---|---|---|
| 3-5 mm | 70% girdle | 0.2 mm | Hold upside down |
| 6-8 mm | 65% girdle | 0.3 mm | Hold upside down |
| 9+ mm | 60% girdle | 0.4 mm | Hold upside down |
We pair this with prong and basket geometry notes for designers who mix tension and prong elements on one ring. Larger stones get shallower seats because more metal pressure acts on a bigger girdle; the clamp itself is tuned by the shank width, not by seat depth.
Wear expectation for tension settings
Tension rings rely on the shank staying springy. Over 10 to 15 years the metal work-hardens and relaxes slightly, and the stone can rattle. We tell customers to bring the ring in annually for a clamp check, and we sell a tension re-tighten service for our wholesale partners. We also warn buyers that tension settings are not suitable for stones with internal cleavage planes (some emeralds, many sapphires); a prong bezel is safer there. We will flag that on the quote when we see the stone spec.
Frequently Asked Questions
Can any stone be set in tension?
No. Stones with cleavage planes, inclusions near the girdle, or extreme thin girdles should be prong or bezel set. Diamonds, moissanite and most rubies in good condition work.
What happens if the stone drops out?
It almost always means the seat was cut too shallow or the shank relaxed. We re-cut the seat and re-set under warranty for the first 12 months on manufacturing looseness.
Is a tension ring easy to resize?
No. Resizing changes the shank's spring and ruins the clamp. Most tension rings are remade rather than resized. We tell customers this at purchase.
How deep is a tension seat compared to a prong seat?
Shallow. A prong seat can go to 90% of girdle because the prongs bend over. A tension seat stops at 60-70% because the clamp is the only thing holding.
Can I design a tension ring in CAD myself?
Yes, but send the CAD to us before printing. We flag seat depths that will crack the stone and suggest the undercut before any wax is printed.
Key takeaways
- Measure the actual stone; cut the seat to 60-70% of real girdle thickness.
- Undercut the pavilion so it does not bottom out and lift the stone.
- Dummy-set a resin replica before the real stone touches the ring.
Next step: send us your tension CAD and the stone spec sheet. We return a seat-depth recommendation and a dummy-set photo before casting. Custom manufacturing opens at 15 pieces per tension design, and we release bulk only after a sample ring passes our dummy and real-stone test.
