Purity, and what the remaining few per cent does. The 96 per cent grade carries a glassy boundary phase that makes it cheaper to form, easier to metallise and slightly more forgiving. The 99.5 per cent grade is harder, whiter, a better high-temperature insulator and more chemically resistant. For most wear and insulation duties 96 per cent is sufficient and noticeably cheaper.

Only by diamond grinding, and it is slow and expensive. Fired alumina is close to the hardness of sapphire, so conventional cutting tools do nothing. Parts are formed and machined green, then fired, with the firing shrinkage allowed for. Where a part must be cut to size on ordinary equipment, machinable glass ceramic exists for exactly that reason.

Poorly, relative to the non-oxide ceramics. Its combination of high thermal expansion and moderate conductivity sets up steep internal stresses when temperature changes quickly, and because it cannot yield it fails by cracking without warning. Gradual ramping is essential in furnace use, and genuinely shock-prone duty belongs to silicon carbide or silicon nitride.

This is one of its principal uses. Alumina maintains high volume resistivity and dielectric strength far above the point where polymers have decomposed, which is why it is standard for thermocouple sheaths, feedthroughs, heater supports and electronic substrates. Higher purity grades hold that resistivity better as temperature climbs.

Very broad. It resists most mineral acids, alkalis, solvents and molten metals, which is why it is used for pump and valve components in aggressive service. The notable exceptions are hydrofluoric acid and strong hot caustic, which attack it, and the glassy phase in lower-purity grades is the first thing to be attacked.

Rod, tube, plate, sheet, washers, balls and blanks, in standard diameters and thicknesses intended to be finished by grinding. Because post-fire machining is expensive, stock sizes are chosen to leave minimal grinding allowance, and it is usually cheaper to design around an available size than to grind a large one down.

Alumina is substantially cheaper and harder; zirconia is far tougher and less likely to chip or fracture from impact. Where the failure mode is abrasive wear, alumina is the better value. Where it is cracking, edge chipping or impact, zirconia earns its price. They are not interchangeable on price alone.