A cone bur — also called a tapered bur — is a rotary cutting tool with a conical profile, used by jewellers and stone setters to prepare the angled seat inside a drilled hole that will receive a faceted gemstone. By bevelling the interior wall of the hole to a precise angle, the cone bur creates a ledge that supports the stone's girdle, distributing bearing pressure evenly around the circumference and preventing the culet from contacting the base of the setting. It is among the most fundamental tools in the setter's kit, indispensable for bezel, gypsy, and flush-setting work.

Form and Construction

Cone burs are manufactured in high-speed steel (HSS) or tungsten carbide. Carbide variants hold a sharper edge considerably longer and are preferred for production work or when cutting harder alloys such as platinum and white gold, which are notably more resistant than yellow gold or silver. The cutting head tapers from a flat or slightly rounded tip to a wider shoulder, with fluted cutting edges running along the taper. Shank diameters are typically 2.35 mm (the standard for most flexible-shaft handpieces and pendant drills) or 3 mm for heavier bench-motor use.

Taper angle and head diameter are the two critical variables. Head diameters are available in incremental steps — commonly ranging from under 1 mm to 5 mm or more — allowing the setter to match the bur closely to the stone's girdle diameter. Taper angles vary by manufacturer but are generally designed to complement the pavilion angle of standard round brilliants and other common cuts; a shallower cone suits a flatter pavilion, while a steeper cone accommodates deeper stones.

Function in Seat Preparation

Before a cone bur is introduced, a pilot hole is typically drilled through or partially into the metal using a twist drill or hart bur. The cone bur is then run at moderate speed — excessive speed generates heat that can work-harden the metal and dull the bur prematurely — and advanced into the hole with light, controlled pressure. The result is a countersunk seat: a smooth, angled recess whose diameter at the top matches the stone's girdle and whose angle supports the girdle facets or the girdle edge itself.

Correct seat depth is critical. If the seat is cut too shallow, the stone will sit proud of the metal surface and the setting metal cannot be pushed over the girdle cleanly. If cut too deep, the stone drops below the intended level and the culet may be exposed to damage, or the girdle may be unsupported. Experienced setters test the fit repeatedly, lowering the stone into the seat by hand to check that it sits level, with no rocking, and at the correct height relative to the surrounding metal.

Relation to Other Setting Burs

The cone bur belongs to a broader family of setting burs, each suited to a different stage of preparation or a different setting style. Hart burs (also called bearing burs) cut a narrow, precise ledge for prong settings; ball burs open and round the base of a hole; and cup burs finish the tips of prongs after setting. The cone bur is distinguished by its versatility — its tapered geometry makes it the preferred choice wherever a smooth, angled seat must be cut to receive the girdle of a stone in a closed or semi-closed setting.

Care and Maintenance

Cone burs are consumable tools. HSS burs blunt relatively quickly in hard metals and should be replaced rather than forced once cutting efficiency declines, as a dull bur tends to chatter and produce an uneven seat. Carbide burs are more durable but brittle; they should not be used at angles that impose lateral stress on the shank, which risks snapping the head. Burs should be kept clean of metal swarf and stored so that cutting edges do not contact one another. A light application of bur lubricant or cutting wax reduces friction and extends working life in both HSS and carbide grades.