One-micron diamond paste is a precision abrasive compound in which synthetic diamond particles of one micrometre (1 µm) average diameter are suspended in a carrier medium — typically a water-soluble or oil-based gel — and used in lapidary faceting to bridge the critical gap between coarse material removal and true optical polishing. At this particle size, the compound removes the fine scratches and sub-surface damage left by coarser pre-polishing laps (commonly 3 µm or 8 µm) while itself leaving a surface refined enough to accept final polishes in the 0.25 µm or 0.1 µm range. In most professional faceting sequences, the 1 µm stage is regarded as the standard transition point at which the workpiece moves from the cutting regime into the polishing regime.

Particle Size and Surface Mechanics

Diamond abrasives are classified by the mean diameter of their particles, expressed in micrometres (µm) or, in older trade usage, in mesh grades. One micrometre equals one-thousandth of a millimetre. At this scale, individual diamond crystallites act as micro-cutting tools, each ploughing a channel whose depth is a function of the particle radius, applied pressure, and the hardness differential between abrasive and workpiece. On a hard gemstone such as corundum (Mohs 9) or chrysoberyl (Mohs 8.5), a 1 µm particle produces scratch channels shallow enough that subsequent polishing with sub-micron compounds can eliminate them within a practical working time. Coarser compounds — 3 µm and above — leave channels too deep for fine polishes to erase efficiently; finer compounds — 0.5 µm and below — lack the cutting rate to remove 3 µm scratches in any reasonable time. The 1 µm grade thus occupies a necessary functional position in the sequence rather than being an optional refinement.

Lap Substrates

The choice of lap material used with 1 µm paste significantly influences the character of the surface produced. Common substrates include:

• Leather laps — vegetable-tanned or chrome-tanned leather provides a semi-resilient surface that conforms slightly to facet geometry, distributing abrasive action evenly. Widely used for corundum, spinel, and chrysoberyl.
• Phenolic (Lucite or Corian-type) laps — harder and less yielding than leather, these produce a flatter facet with less risk of rounding edges, preferred when geometric precision is paramount.
• Wood laps — close-grained hardwoods such as maple are occasionally used, particularly in older workshop traditions; they hold paste well but can introduce directional grain effects at very fine particle sizes.
• Tin and lead-alloy laps — soft metal laps charged with 1 µm paste are employed for very hard materials and for achieving a near-mirror surface before transitioning to oxide polishes.

The paste is applied sparingly — typically a few millimetres extruded from a syringe — and worked with a small quantity of the appropriate extender (water or oil, matched to the carrier type) to maintain an even film across the lap surface.

Gemstone Suitability

One-micron diamond paste is effective across a wide range of gem species, but its utility is greatest on materials of Mohs hardness 8 and above, where oxide polishes (cerium oxide, aluminium oxide, chromium oxide) may struggle to cut efficiently. Principal applications include:

• Corundum (ruby and sapphire, Mohs 9) — 1 µm paste on a leather or phenolic lap is a standard pre-polish stage before final treatment with 0.25 µm diamond or aluminium oxide on a tin lap.
• Chrysoberyl (including alexandrite and cat's-eye, Mohs 8.5) — responds well to the 1 µm stage; the compound's cutting action is sufficient to remove 3 µm scratches without over-cutting the relatively brittle cleavage directions.
• Spinel (Mohs 8) — 1 µm paste produces an excellent pre-polish surface; spinel's isometric crystal structure means it polishes uniformly in all orientations.
• Topaz (Mohs 8, with perfect basal cleavage) — used with caution; the 1 µm stage is appropriate but pressure and lap speed must be moderated to avoid cleavage initiation.
• Garnet species (Mohs 6.5–7.5) — softer garnets such as almandine and pyrope may be taken directly to 0.5 µm or oxide polishes, but harder varieties such as demantoid and tsavorite benefit from the 1 µm stage.

On softer materials (quartz, feldspar, fluorite), the 1 µm stage is generally unnecessary; oxide polishes alone are sufficient and the diamond compound's cost is not justified.

Position in the Faceting Sequence

A typical professional faceting sequence for a hard gemstone proceeds through discrete abrasive grades, each removing the damage layer introduced by the previous stage. A representative sequence might read: 260 mesh (coarse shaping) → 600 mesh (fine shaping) → 1200 mesh (pre-polish grinding) → 3 µm diamond paste → 1 µm diamond paste → 0.25 µm diamond paste or aluminium oxide polish. Some cutters insert an 8 µm stage between the 1200-mesh lap and the 3 µm paste, particularly for very hard or large stones. The 1 µm stage typically requires two to five minutes per facet on corundum, depending on lap condition, paste freshness, and the depth of scratches inherited from the previous stage. Visual inspection under oblique fibre-optic illumination at 10× magnification is the standard method for confirming that all 3 µm scratches have been eliminated before advancing.

Handling and Storage

Diamond paste syringes should be stored capped and away from direct heat; the carrier medium can dry or separate if the syringe tip is left open between sessions. Cross-contamination between grades — the introduction of even a few 3 µm particles onto a 1 µm lap — is the most common cause of persistent scratching at the polishing stage and is avoided by maintaining dedicated laps and applicators for each grade, clearly labelled. Water-soluble pastes are cleaned from laps with distilled water; oil-based pastes require a light solvent such as isopropyl alcohol.