A hart bur, also known as an underbezel bur, is a specialised rotary cutting tool used in stone setting to mill an undercut ledge — or bearing seat — within a drilled or pierced hole in metal. Its defining feature is a 90-degree shoulder profile that cuts laterally beneath the surface of the metal as the bur rotates, producing a recessed shelf on which the girdle of a gemstone can rest. Once the stone is seated on this ledge, the metal above is pushed or burnished inward to secure it, whether in a bezel, flush, or rub-over setting. Without the undercut, a stone set into a straight-walled hole risks either falling through or lifting proud of the surface; the hart bur eliminates both failure modes.

Form and Construction

Hart burs are manufactured from high-speed steel or tungsten carbide and are turned in a standard flex-shaft handpiece or pendant drill. The cutting head presents a stepped, shoulder-like geometry: the lower portion is narrower and clears the base of the hole, while the upper cutting edge projects outward at a right angle to score the undercut channel. This profile distinguishes the hart bur from a setting bur (also called a cup bur or bearing bur), which produces a conical seat matched to a faceted pavilion, and from a plain twist or cylinder bur, which cuts straight walls without a shoulder.

The tools are supplied in graduated diameters, typically measured in tenths of a millimetre, to correspond with standard gemstone girdle diameters. A setter will select a hart bur whose shoulder diameter matches — or fractionally undercuts — the girdle diameter of the stone being set, ensuring a snug mechanical fit with minimal rocking.

Application in Setting

Hart burs are most commonly employed in bezel setting and flush (or gypsy) setting, particularly when working with round or calibrated stones in yellow gold, white gold, platinum, or silver. The typical workflow proceeds as follows:

• A pilot hole is drilled through or into the metal to a depth slightly shallower than the stone's total depth.
• The hart bur, matched to the stone's girdle diameter, is introduced into the hole and rotated at moderate speed to cut the undercut ledge at the correct depth.
• The stone is test-seated; the girdle should drop onto the shelf and sit level, with the table either flush with or fractionally below the metal surface depending on the setting style.
• The metal above the ledge is then pushed over the girdle using a bezel pusher, burnisher, or hammer handpiece, locking the stone in place.

Because the undercut is invisible once the stone is set, the hart bur's work is entirely structural rather than decorative — yet it is fundamental to the integrity of the finished piece. A poorly cut or mismatched undercut is among the most common causes of stones becoming loose in flush and bezel settings over time.

Relationship to Other Setting Burs

The hart bur belongs to the broader family of setting burs, each designed to prepare a specific type of seat. Where a ball bur or cup bur cuts a curved, conical recess suited to the pavilion of a round brilliant, the hart bur cuts a flat horizontal shelf suited to the girdle. Some setters use both in sequence: a cup bur first to relieve the pavilion, then a hart bur to define the girdle seat precisely. In trade parlance, the undercut produced by the hart bur is often called an undercut seat or bearing, and achieving a clean, level bearing is considered a mark of competent bench work.

In the Trade

Hart burs are standard inventory items at jewellery tool suppliers worldwide and are available individually or in graduated sets. Tungsten carbide versions, though more expensive than high-speed steel, hold their cutting edge significantly longer and are preferred when working in platinum or hard alloys. Setters working in production environments — where dozens of identical stones must be set to a consistent depth — often rely on depth stops or collar guides fitted to the handpiece to ensure uniform undercut depth across a run of pieces.