Aluminium Oxide Polish
Aluminium Oxide Polish
A precision abrasive for hard gemstones and optical surfaces
Aluminium oxide polish is a fine-grained abrasive compound composed of aluminium oxide (Al₂O₃), used in the final polishing stage of gem cutting to produce a high-lustre surface on hard gemstones. It is particularly suited to corundum (sapphire and ruby), spinel, and chrysoberyl — materials in the Mohs 8–9 range — where softer polishing agents such as cerium oxide or tin oxide prove insufficiently aggressive or leave a suboptimal finish. The compound is supplied as a dry powder or aqueous slurry and applied on a range of lap surfaces including leather, felt, and hardwood.
Composition and Particle Size
The abrasive action of aluminium oxide polish derives from the extreme hardness of the Al₂O₃ crystal lattice (Mohs 9), which allows it to abrade and burnish surfaces of nearly equal hardness without the deep scratching associated with coarser diamond compounds. Commercial polishing grades are characterised primarily by mean particle size. The two most widely recognised formulations in lapidary and optical work are Linde A, with a mean particle diameter of approximately 0.3 micrometres, and Linde B, with a mean particle diameter of approximately 0.05 micrometres. Linde A is the more commonly used of the two for gemstone work; Linde B, being finer, finds application where an exceptionally high surface quality is required, such as in optical component finishing.
Both grades were originally developed by the Linde Division of Union Carbide Corporation for precision optical polishing, and their adoption by the lapidary trade followed naturally from their proven performance on hard oxide minerals.
Lap Materials and Application
The choice of lap material significantly influences the polishing result. For corundum and spinel, a hard leather lap — typically charged with a dilute slurry of Linde A in water — is the most widely used combination, producing a bright, well-defined polish with minimal rounding of facet edges. Felt laps offer a softer action and are sometimes preferred for slightly lower hardness stones or for achieving a more reflective surface at the cost of some edge definition. Hardwood laps (often maple or similar close-grained species) charged with aluminium oxide are used by some cutters for their intermediate hardness and the degree of control they afford.
The slurry concentration is kept dilute — a small quantity of powder in distilled water — to prevent the buildup of dried abrasive that can introduce scratches. Lap speed, pressure, and the direction of stroke relative to the facet all affect the final result, and experienced lapidaries adjust these variables according to the specific material being polished.
Gemstones Suited to Aluminium Oxide Polish
Aluminium oxide polish is most effective on gemstones whose hardness is close to that of the abrasive itself. Principal applications include:
- Corundum (ruby and sapphire, Mohs 9) — the primary use case, where aluminium oxide polish is considered the standard finishing agent by most professional cutters.
- Spinel (Mohs 8) — responds well to Linde A on leather.
- Chrysoberyl (Mohs 8.5), including alexandrite and cat's-eye chrysoberyl.
- Topaz (Mohs 8) — though the perfect basal cleavage of topaz requires careful technique to avoid cleavage-related surface disruption.
On softer materials (quartz and below), aluminium oxide polish can be used but is generally considered unnecessarily hard; cerium oxide or other agents are typically preferred for those species.
Relationship to Diamond Compounds
In modern lapidary practice, aluminium oxide polish is used exclusively at the final polishing stage, after pre-polishing with progressively finer diamond compounds (typically concluding at 3,000 or 8,000 grit diamond). The transition from diamond to aluminium oxide represents a shift in mechanism: diamond laps cut and abrade, while aluminium oxide on a compliant lap polishes by a combination of fine abrasion and a degree of surface flow or burnishing. The result is a surface quality that diamond alone, on a hard lap, does not consistently achieve on corundum.