Grit by grit: how grinding works
The logic of a grit progression — why you cannot skip stages and what water does for the stone.
Grinding is material removal. Each wheel leaves a characteristic depth of scratch on the stone surface. The logic of the grit progression: start coarse to shape efficiently, then each successive finer wheel removes only the scratches the previous wheel made. Skip a grit and the finer wheel has to remove scratches too deep for it to handle efficiently — you spend much more time, or never fully clean up the surface.
For opal, a typical progression starts at 100 or 220 grit for shaping the dome, then 400, 600 or 800, 1200, and 3000 for pre-polish. The specific wheels depend on the setup, but the principle is the same at every stage: do not move to the next grit until the current stage is fully complete — no bright lines under raking light from the previous wheel.
What water does
Every grinding stage runs wet. Water cools the stone surface — heat from grinding is a real hazard for opal, which crazes under thermal shock — and it carries the ground material away from the wheel so it does not clog the abrasive surface. Dry-grinding even briefly can produce a frosted zone or stress crack that is permanent. Keep water flowing across the wheel throughout.
Pressure: opal is a soft gem and needs lighter contact against the wheel than harder stones. Excessive pressure causes chipping at the girdle edge and can crack the stone under the dome. Let the wheel do the work. Your job is to move the stone consistently across the wheel face — a slow, deliberate orbital motion — not to press hard. More pressure does not mean faster cutting with opal; it means more risk.