Colour graining in gemmology denotes a growth-related internal feature in which a crystal shows visible bands, planes, or zones of slightly different colour saturation, hue, or transparency, traceable to variations in growth conditions, trace-element incorporation, or post-growth deformation. In diamond the term is most commonly applied to the linear or planar features described on GIA grading reports as "colored graining" or "internal graining", and it carries specific meaning within the diamond clarity-grading vocabulary. In coloured stones, particularly in corundum and beryl, the analogous feature is more often called colour zoning, but the underlying phenomenon is the same: the crystal recorded its own growth history, and that history is visible as variation in the finished stone.

In diamond

Diamond colour graining is observed under magnification as faintly visible bands, lines, or planes within the stone, sometimes brown, sometimes pinkish, sometimes simply a slightly different shade of the body colour. The feature is caused by minor variations in nitrogen incorporation or in plastic deformation along the diamond's growth or post-growth history, and it is more common in fancy-coloured diamonds, particularly pinks and browns, where it can be a contributor to the overall body colour. On a GIA grading report, internal graining that is not visible face-up at 10× magnification is typically not plotted; visible graining contributes to the clarity grade and is noted as an inclusion characteristic. "Graining" without modifier on a GIA report typically refers to colourless graining (also called internal graining), a transparent textural feature; "coloured graining" specifies a feature with visible colour and is more diagnostic of certain stones, particularly type IIa pinks where pink graining can constitute the entire colour mechanism.

In corundum and beryl

In sapphire and ruby the analogous feature is colour zoning, in which the crystal shows alternating bands of stronger and weaker colour, often parallel to the prism faces of the original hexagonal crystal. Sri Lankan and Madagascar sapphires frequently show such zoning, and the orientation of the cut to mask or reveal it is a key cutter's decision. In emerald, beryl colour zoning is common and is generally accommodated by the cutter rather than masked. Aquamarine, heliodor, and morganite all show comparable features.

Diagnostic and grading consequences

Colour graining is a useful identification feature: in diamond, strong pink graining is suggestive of natural type IIa pink diamond and tends to be absent in laboratory-grown HPHT or CVD pinks, although the boundaries are not absolute and laboratories rely on multiple lines of evidence. In sapphire, the presence and orientation of growth-related colour zoning helps to distinguish natural from synthetic stones, since flame-fusion (Verneuil) synthetic sapphires show curved colour banding rather than the straight, hexagonally-oriented zoning of natural material. In flux-grown synthetic corundum the colour zoning patterns are different again. The feature is therefore one of the routine items checked under the microscope by laboratory gemmologists.

Aesthetic implications

For the buyer, visible colour graining is generally a slight detriment, more so when it is concentrated in the face-up view than when it is confined to the pavilion or the girdle area. In sapphires, strong colour zoning can produce stones that show a saturated colour from one direction and a weak colour from another, and the cutter's task is to orient the table such that the strongest colour zone is foremost. In diamond, faint internal graining is generally tolerated at higher clarity grades, and visible coloured graining tends to push the stone into a lower grade or, in fancy colour grading, contributes to the named hue.