Stone Setting and Gemstone Handling
What is examined
This domain tests gemstone properties, setting selection, burr choice, seat cutting, tightening and stone safety. It also checks whether you can handle stones without loss or damage and match cleaning and heat exposure to the gemstone.
Core rules
- Mohs hardness: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, feldspar 6, quartz 7, topaz 8, corundum 9, diamond 10. Hardness is scratch resistance, not toughness; emerald and opal are brittle or sensitive despite moderate hardness.
- Heat sensitivity: opal, pearl, amber, coral, tanzanite, emerald and many treated stones can be damaged by direct heat or ultrasonic cleaning. Remove sensitive stones before soldering or use a heat shield and wet tissue, but removal is safer.
- Setting types: bezel (rubover), prong (claw), pavé, channel, flush (gypsy), tension and invisible settings. Each needs a different seat and tightening method.
- Burrs: setting burr cuts a conical seat with a bearing; hart burr undercuts for pavé and channel work; bud burr enlarges a hole; round burr hollows a seat. Select the burr by stone diameter and girdle thickness.
- Setting sequence: measure the stone, choose the burr, cut the seat, check the stone sits at the correct depth with the girdle just above the bearing, tighten prongs alternately, then check security with a probe and loupe.
- Handling: work over a tray, keep stones in labelled papers or compartments, never place loose stones on the bench top, and account for every stone before and after the job.
Worked example
Set a 3.00 mm round brilliant in a four-claw setting. Measure the stone and select a 3.00 mm setting burr. Cut the seat until the girdle sits just above the bearing surface, checking the fit frequently. Raise and adjust the prongs, file the prong tips to shape, then push them over the stone alternately, not all in one direction. Burnish the tips, check with a loupe that each claw contacts the crown and that the stone is not rocked, then clean carefully. The reasoning is that the seat controls depth and the prongs control security; cutting too deep or over-tightening can chip the girdle.
Traps
Using a burr that is too large or too small; cutting the seat too deep; over-tightening prongs and cracking the stone; heating a sensitive stone; putting opal, pearl or emerald in an ultrasonic cleaner; failing to check stone security after setting; losing stones because they were not kept in a tray; polishing over a stone with a mop that catches a prong.