A drill bit is a rotary cutting tool with helical flutes that evacuate swarf as it bores a cylindrical hole through a workpiece. At the jeweller's bench, drill bits span an enormous range of materials and scales — from high-speed steel twist drills used on precious metals down to sub-millimetre diamond-tipped core drills capable of piercing corundum or spinel without shattering the stone. The choice of bit type, rotational speed, feed pressure, and cooling method determines whether the result is a clean, precise bore or a cracked and ruined specimen.

Types Used in Jewellery and Lapidary Work

Three principal categories of drill bit appear at the jewellery bench and in the lapidary workshop:

• High-speed steel (HSS) twist drills. The standard spiral-fluted drill, manufactured from high-speed steel (typically M2 or M35 grade), is entirely adequate for boring holes in gold, silver, platinum, and base metals. Jewellers work with sets ranging from approximately 0.3 mm to 3.0 mm in diameter. The twist geometry — a pair of cutting lips at the tip and helical flutes along the shank — simultaneously cuts and clears chips. Lubrication with a cutting fluid or beeswax prolongs edge life and reduces heat build-up in the metal.
• Carbide-tipped and solid carbide drills. Tungsten carbide offers substantially greater hardness than HSS and is preferred when drilling hardened steel findings, titanium, or very dense alloys. Solid carbide micro-drills, available in sizes below 0.5 mm, are also used for fine piercing work in watch-part manufacture and high-precision settings.
• Diamond-impregnated core drills and solid diamond bits. Drilling gemstones — whether for bead stringing, inlay work, or the creation of drilled pendants — requires abrasive action rather than conventional cutting. Diamond core drills consist of a thin-walled steel tube whose rim is bonded with industrial diamond particles; the hollow centre allows a plug of material to be freed cleanly. Solid diamond-tipped bits (sometimes called sintered diamond drills) are used for smaller diameters. Both types demand continuous water cooling to dissipate frictional heat, which would otherwise cause thermal shock and internal fracturing in brittle materials such as emerald, opal, or tanzanite.

Drilling Gemstones: Key Considerations

Gemstone drilling is among the most technically demanding operations in the lapidary workshop. Several factors govern success:

• Hardness and toughness. Mohs hardness dictates the grade of diamond abrasive required, but toughness — resistance to cleavage and fracture — is equally critical. Topaz, for example, has perfect basal cleavage and requires exceptionally gentle feed pressure even though its hardness (8 on the Mohs scale) is well within the capability of standard diamond drills. Jade (jadeite and nephrite), by contrast, is tough enough to tolerate more assertive drilling.
• Water cooling. Continuous water immersion or a drip feed directed at the drill tip is non-negotiable for most gemstones. Without it, localised temperatures rise rapidly, causing differential thermal expansion that fractures the stone from within. Some lapidaries drill in a shallow tray of water, keeping the stone submerged throughout the operation.
• Speed and pressure. Diamond drills for gemstones operate most efficiently at relatively low rotational speeds — typically 1,000–3,000 rpm for core drills on harder materials — combined with light, consistent feed pressure. Excessive speed generates heat faster than water can dissipate it; excessive pressure risks chipping the entry and exit points of the bore.
• Entry and exit chipping. The surface where the drill breaks through is particularly vulnerable. Drilling halfway from each face, meeting in the middle, is a standard technique for minimising exit chipping on faceted or polished stones.

Equipment and Setup

In professional jewellery workshops, drill bits are mounted in a pendant drill (flexible-shaft machine), a dedicated drill press, or a high-speed rotary tool such as a Foredom or similar handpiece. A drill press offers the advantage of a controlled, vertical feed with adjustable depth stops, which is particularly valuable when drilling to a precise depth without breakthrough. For gemstone work, purpose-built lapidary drill presses with water reservoirs are available, though many craftspeople improvise effectively with a standard drill press and a shallow water bath secured to the table.

Bit shanks are standardised: 2.35 mm (3/32 inch) shanks fit most jeweller's handpieces, while straight shanks of 3.0 mm or larger suit bench drill chucks. Collet-style chucks provide superior concentricity for very fine bits, reducing the wobble that can enlarge a bore or snap a sub-millimetre drill.

Maintenance and Replacement

HSS bits dull with use and can be resharpened on a bench grinder or dedicated drill-sharpening jig, though at sub-millimetre diameters replacement is generally more practical than resharpening. Diamond core drills wear by the gradual loss of exposed diamond particles from the bonding matrix; a worn drill that has glazed over can sometimes be re-dressed by briefly running it against a soft abrasive such as a piece of sandstone or aluminium oxide block, which exposes fresh diamond. Carbide bits are brittle and should never be subjected to lateral stress; snapping a carbide micro-drill in a workpiece is a common and costly error when feed pressure is applied unevenly.