A jeweler in Toronto set a 6.5 mm round brilliant moissanite into a pre-made cast seat. The stone sat level, the prongs closed, and it looked perfect. Ten days later the stone sat cock-eyed: one prong had lifted because the girdle had rested on a high spot in the seat rather than on the seat itself. The stone was not loose, but it was not supported, and over daily wear the prong work cracked. The seat and the girdle are the contact that decides whether a setting survives two years or two weeks.

A cast seat is a rough cup cut into the wax or milled into metal. It is not the final bearing surface. Before a stone goes in, the seat has to be opened to the exact girdle diameter and angled so the girdle sits fully on the metal, not on a point. This guide covers the seat dimensions, the burr progression, and the tolerance that wholesale setters work to.

Girdle diameter vs seat diameter

The seat should open to roughly 0.1-0.15 mm smaller than the stone's girdle diameter, so the metal slightly grips the stone's widest point. If the seat is cut to full girdle diameter or larger, the stone drops in and tilts; the prongs then hold it by pressure alone instead of by a ledge. For a 6.5 mm round stone, the target seat bore is about 6.35-6.40 mm.

The seat angle matters as much as the bore. A round brilliant girdle is a thin edge; the seat should be angled at about 45 degrees so the girdle rests on a small flat, not on a sharp corner. A sharp-cornered seat cracks the girdle on harder stones and leaves a contact that loosens over time. For stone grading reference, see GIA 4Cs.

The burr progression

Setting burrs (bud burrs, setting burrs, Hart burrs) come in sizes matched to the stone. The sequence on a cast seat:

- Open the rough cup with a round bud burr slightly smaller than the girdle.
- Switch to a setting burr (a flat-sided, angled burr) of the correct stone size to cut the bearing ledge.
- Test-fit the stone; it should rest level with even contact all around.
- Refine with a small cup burr or wheel if one quadrant sits high.

The setter checks the seat under a microscope or 10x loupe: look through the top of the stone. A bright, even ring of contact around the girdle means full support; dark gaps mean the stone is resting on one point.

Why cast seats need hand opening

CAD and investment casting can produce a seat close to size, but wax shrinks during casting and the cast seat runs 0.1-0.2 mm small and irregular. A factory that skips hand opening and pushes the stone into the cast seat is counting on prong pressure alone. On a tennis bracelet with 3 mm stones, that tolerance is forgiving; on a 2 ct solitaire, a 0.1 mm uneven seat translates to a visible wobble within a year.

For wholesale buyers, ask whether the setter opens each seat by hand after casting. The photo to request is a top-down loupe shot of a set stone showing even girdle contact. See also bezel finishing and tennis bracelet stone counts.

How much smaller should the seat be than the stone?

About 0.1-0.15 mm smaller than the girdle diameter, so the metal ledge slightly grips the widest point. For a 6.5 mm stone, target 6.35-6.40 mm.

Why does my stone sit tilted after setting?

The seat was cut too large or on a sharp corner, so the girdle rests on one point. Re-open the seat with a setting burr to create a flat bearing ledge.

Can CAD cast seats be used as-is?

No. Wax shrinkage leaves the seat 0.1-0.2 mm small and irregular; hand opening with a setting burr is required for durable support.

How do I inspect seat contact as a buyer?

Look through the stone under a 10x loupe. You should see an even, bright ring of metal touching the girdle all around; dark gaps mean point contact.

A correct seat costs 30-60 seconds of setter time per stone and is the cheapest insurance agai

Setting tolerance and the reorder

A seat opened by hand has a tolerance of about 0.05-0.1 mm per stone. On a reorder of the same design, that tolerance stacks: if the first run's seats were opened slightly tight, the second run's stones will also sit tight. Buyers who approve a first sample with a firm but not loose stone should not expect the reorder to feel identical unless the setter works to the same gauge. Asking for a stone-size tolerance on the work order (e.g., seat bore 6.35-6.40 mm for a 6.5 mm stone) keeps the setter from drifting across runs.

nst loose-stone returns. We hand-open every seat on solitaire and three-stone pieces and document girdle contact on sample approval. If you are building a set line, plan your earring SKUs and review MOQ tiers before the next production slot.